Python 3.4 support

- Python 3.4 now tested when running tox
- Travis-CI testing for 3.4 will be enabled when available
Maintain hierarchy in api, enforce with

 - Model (now Network) is recursively structured.
 - 'with' is required.
 - Fixed a test which occassionally failed due to floating point
   comparisons
 - NengoObject base class for simplifying __hash__, __str__, etc...
 - A bit of general cleanup with string quotes and specifiers
Improved test_builder