Added general API caller.
The general API caller can call APIs directly by API path.
It get more flexible to support new APIs in the future.
Moved GeneralCaller from jplurk_oauth.module to jplurk_oauth.ext
package.
Suggest ONLY use this class when new API is not implemented.
Merge pull request #7 from askeing/general_API_caller

General api caller. 
Suggest ONLY use this class when new API is not implemented.