refactors OpenTokHttpClient so it is no longer a Singleton (for testing)

*  adds a fully isolating test for creating default sessions #11
*  fixes #13, the User-Agent was never set correctly (discovered in testing)
*  gives more responsibility to OpenTokHttpClient and makes it a subclass of
   AsyncHttpClient
*  uses RequestFilter subclass PartherAuthRequestFilter to add headers
*  OpenTokHttpClient how has a builder pattern initialization API
*  fixes #18, apiUrl is now part of the initialization