Add Foursquare2 API and example

src/main/java/org/scribe/builder/api/Foursquare2Api.java

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+package org.scribe.builder.api;
+
+import org.scribe.extractors.*;
+import org.scribe.model.*;
+import org.scribe.utils.*;
+
+public class Foursquare2Api extends DefaultApi20
+{
+  private static final String AUTHORIZATION_URL = "https://foursquare.com/oauth2/authenticate?client_id=%s&response_type=code&redirect_uri=%s";
+
+  @Override
+  public String getAccessTokenEndpoint()
+  {
+    return "https://foursquare.com/oauth2/access_token?grant_type=authorization_code";
+  }
+
+  @Override
+  public String getAuthorizationUrl(OAuthConfig config)
+  {
+    Preconditions.checkValidUrl(config.getCallback(), "Must provide a valid url as callback. Foursquare2 does not support OOB");
+    return String.format(AUTHORIZATION_URL, config.getApiKey(), URLUtils.formURLEncode(config.getCallback()));
+  }
+
+  @Override
+  public AccessTokenExtractor getAccessTokenExtractor()
+  {
+    return new JsonTokenExtractor();
+  }
+}

src/main/java/org/scribe/extractors/FoursquareAccessTokenExtractor.java

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+package org.scribe.extractors;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.scribe.exceptions.OAuthException;
+import org.scribe.extractors.TokenExtractor20Impl;
+import org.scribe.model.Token;
+import org.scribe.utils.Preconditions;
+import org.scribe.utils.URLUtils;
+
+public class FoursquareAccessTokenExtractor extends TokenExtractor20Impl {
+
+    private static final String TOKEN_REGEX = ".[\\s]+\"access_token\":\"(\\S*?)(&(\\S*))?\"[\\s]+.";
+    private static final String EMPTY_SECRET = "";
+
+    public Token extract(String response) {
+        Preconditions.checkEmptyString(response, "Response body is incorrect. Can't extract a token from an empty string");
+
+        Matcher matcher = Pattern.compile(TOKEN_REGEX).matcher(response);
+        if (matcher.matches()) {
+            String token = URLUtils.formURLDecode(matcher.group(1));
+            return new Token(token, EMPTY_SECRET);
+        } else {
+            throw new OAuthException("Response body is incorrect. Can't extract a token from this: '" + response + "'", null);
+        }
+    }
+}

src/main/java/org/scribe/extractors/JsonTokenExtractor.java

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+package org.scribe.extractors;
+
+import java.util.regex.*;
+
+import org.scribe.exceptions.*;
+import org.scribe.model.*;
+import org.scribe.utils.*;
+
+public class JsonTokenExtractor implements AccessTokenExtractor
+{
+  private Pattern accessTokenPattern = Pattern.compile("\"access_token\":\"(\\S*)\"");
+
+  @Override
+  public Token extract(String response)
+  {
+    Preconditions.checkEmptyString(response, "Cannot extract a token from a null or empty String");
+    Matcher matcher = accessTokenPattern.matcher(response);
+    if(matcher.find())
+    {
+      return new Token(matcher.group(1), "");
+    }
+    else
+    {
+      throw new OAuthException("Cannot extract an acces token. Response was: "+response);
+    }
+  }
+
+}