Finish pyuv_cffi implemention

[veegee]

Currently, most libuv handles are implemented. The remaining tasks are as follows:

• Implement the remaining handle types, and implement those methods which currently raise NotImplementedError.
• Write tests (see the pyuv project for interesting test cases). This includes checking for memory leaks and proper garbage collection. I already did preliminary memory profiling and the current code has no memory leaks on CPython and pypy3, so it's at a good starting point already.
• Make sure it's API-compatible as much as possible with pyuv
• Reorganize the package; currently, all classes are in __init__.py for the sake of simplicity, but this is not the right way to organize any multi-class package in python.