Catch Exceptions thrown by a @Provides method and wrap them in ProvisionException

Regarding issue #35, here's a first attempt into detecting which exceptions are declared by a method that is annotated with `@Provides`.

Two points of interest:

1. Any thrown Exceptions are wrapped in a ProvisionException, but that is not caught yet. I'm not too familiar with the restx codebase to know how to handle them.
2. I've made the assumption that Exceptions do not have type parameters. [`TypeElement#getQualifiedName()`](http://docs.oracle.com/javase/7/docs/api/javax/lang/model/element/TypeElement.html#getQualifiedName%28%29) includes type parameters in its String representation of a thrown Exception.