Configurable Headers for All Requests to PayPal

- Allows adding additional headers to every call to PayPal APIs.
- Refactored OAuthTokenCredential to share code for making Rest Calls
- SDK Config to add headers with http.headers.* syntax
- Fixes #193

sample/sdk_config.ini

@@ -10,6 +10,10 @@ acct1.ClientSecret = EGnHDxD_qRPdaLdZz8iCr8N7_MzF-YHPTkjs6NKYQvQSBngp4PTTVWkPZRb
 [Http]
 http.ConnectionTimeOut = 30
 http.Retry = 1
+
+; Adding HTTP Headers to each request sent to PayPal APIs
+;http.headers.PayPal-Partner-Attribution-Id = 123123123
+
 ;http.Proxy=http://[username:password]@hostname[:port]
 
 ;Service Configuration