Synthesize method signature for dynamic property getters

[imhuntingwabbits]

This PR adds the ability to synthesize the method signature for dynamic property getters which resolves this issue:

#215

There are a couple of issues I foresee:

1. There are no sanity checks, i.e. this PR theoretically allows users to do things they can't today (like mocking CoreData objects) which could be considered evil.
2. There are no checks for read-write / read only properties, so it may be necessary to add signatures for setters to this, but that is unclear to me.

[imhuntingwabbits]

Will do, I'll keep things separated in the future.

[erikdoe]

@imhuntingwabbits Thanks. By the way, did you see #218?