Pycapnp-async

- Timer class `capnp.getTimer()`
- pycapnp is now thread-safe and allows an event loop to be run in each thread
- You must destroy and re-create the event loop to get this functionality (see `test_threads.py`)
- Inheritance now works correctly for interfaces (previously inherited methods were inaccessible from pycapnp)
- Add ability to import modules with dashes or spaces. Use underscores in place of them
- `from_bytes` with builder=True is no longer zero copy. It never worked correctly, and is much safer now
- Add `num_first_segment_words` argument wherever message creation can occur
- Allow restoring a null objectId by passing None to restore
- Support ordered dictionary in `to_dict`
- Add ListSchema class and schemas for native types under `capnp.types` which completes all the Schemas needed to be wrapped. See `test_schema.py` for examples using it
- Add automatic build of C++ libcapnp if it's not detected on the system. Also add flags --force-bundled-libcapnp and --force-system-libcapnp respectively

- Fix build for new 0.21 release of Cython. 0.21 is now the minimum supported version of Cython.

- Fix `to_dict` not converting enums to strings

- Fix compilation problem with gcc 4.8

- Fix problem with uninitialized unions in \_from\_dict
- Add accesible version numbers for C++ libcapnp

- Remove onDrained since it was removed upstream
- Replace usage of strings as enum type with custom `_DynamicEnum` class.
- Also change `Struct.which()` method to be a property `Struct.which` and return an enum type (`_DynamicEnumField`, which behaves much like `_DynamicEnum`).
- TwoPartyServer.run_forever() now will handle more than 1 simulataneous connection.
- Change exception wrapper to detect and raise AttributeError for field lookup exceptions (Fixes problem in Python3.x `__dir__`)
- Allow setting of fields with python dicts.