Update to com.github.scribejava

[ericleong]

This resolves #82, which turned out to be more complicated than I expected. Here are the changes:

• Update org.scribe to com.github.scribejava, the version was updated from 1.3.3 to 2.0.1
• Update the corresponding imports.
• Add async-http-client. Either gson or scribe needs this, and for some reason neither of them includes it.
• Response.getCode() is now final, so it is no longer possible to mock it with Mockito (see model.Request, Reponse objects don't play well with unit test mocking frameworks scribejava/scribejava#313). Luckily, only private methods were being tested, so the solution was to add a helper method that took a code as an additional argument, and then RequestBuilderTest would test that method instead.
• The tests fail on openjdk6 because scribejava targets Java 7 and up: https://github.com/scribejava/scribejava/blob/master/pom.xml#L111-L112