Fix the generic wildcards of the FluentFutures.from methods.

This CL changes the generic arguments for the passed in iterables to the from methods so that they match the corresponding methods in Guavas Futures class. I've also added a (potentially controversial) compiler check class to make sure that these issues don't regress.
Merge pull request #2 from PressAssociation/fluent-futures-from-generics-fix

Fix the generic wildcards of the FluentFutures.from methods.