Create MethodInjector implementation with Reflections

[derrop]

Is your feature request related to a problem? Please describe.
Currently there are the MethodInjectors that are generated with Javassist, but that takes some time to generate. If a MethodInjector should only be used once (for example to invoke a getter which won't be reused anymore), this is unnecessary and reflections can be used.

Describe the solution you'd like
The given interface in MethodInjector.Factory.generate should be implemented with a Proxy and the target method should be invoked via Reflections. This solution will be faster when executed once, but for multiple invocations Javassist should still be used.

Describe alternatives you've considered
The InjectedInvocationHelper that has been removed in this commit could be implemented again to simply invoke a method with a Map of Keys (from Guice) and their values and without creating an interface.