Pycapnp

- 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.

- Remove python 3.2 from travis tests. Python 3.2 still should work fine, but it's more trouble than it's worth to write unicode tests that work in both it and Python2.
- Fix problems with null characters in Text/Data fields. Fixes 19