Renamed proofOfWorkLimit to maxTarget along with get/set and minor refactorings

What changed:

* The proofOfWorkLimit renamed to maxTarget to get it more in line with commonly used terminology (wiki, etc.).
* Some misleading local variables renamed from 'difficulty' to 'target'.

Note to the reviewer:

* These refactorings were fully automated and thus should be safe to merge.
* Breaks public API a bit but almost no one uses the getter being renamed here. Bitcoin Wallet does not, Mycelium does not, and Multibit only once in the tests.
* Tests passs.
* Feel free to discard if the change is not worth it.