Merging InjectorFactory pull request #18 after resolving conflicts

I've merged in @anismiles pull request #18 and fixed the merge conflicts. 

I can understand having an `InjectorFactory` may be against the original design for dropwizard-guice. But it really is quite useful for swapping out the default Guice injector with Governator injector, so that users can take advantage of `@LazySingletons` which in turn would resolve issue #19.
this interface needs to be public, otherwise it will be package-private and no one else can implement it outside `com.hubspot.dropwizard.guice`