Make MPOAuthAPI::initWithCredentials withConfig work.

Previously parts of the code expected the config dictionary to contain NSURL objects and parts expected NSString objects. Consistified code on NSString as that's what's used by the autoloader plist code.
Don't work with result object if there was an error.
Tell linker that we are using ObjectiveC.
Merge pull request #4 from silverorange/fix-initialization

Fix initialization