Added support for Python 3.1+

- Introduces the six library as a dependency and places the used modules
into pyleus.compat.
- Some changes to how objects are mocked to keep things compatible with
Py2 and Py3 versions. Nested context managers had to be changed since 
2.6 doesn’t support the multiple managers in one with stmt and 3.1+ drops
support for the contextlib.nested manager.
- Updated tox.ini and all tests are passing for py26, py27, py31, py32, 
py33, and py34.
updated travis.yml to test the py3 envs
:( travis-ci doesn’t support 3.1
move mock compat from pyleus.compat to pyleus.testing

mock is only a test dependency so it is separated from the rest of the 
2to3 compat modules in pyleus.compat
Merge branch 'develop' into python3
move builtins from pyleus.compat to pyleus.testing

builtins is only used in mocking builtins.open in the testing code
fix for python3 where dict_keys objects do not support indexes
added list coercion for dict.keys() in examples
Merge pull request #50 from justquick/python3

Added support for Python 3

Compatibility issues with Storm mean that Python 3 topologies remain
unsupported, but this branch ensures Pyleus's code itself is valid
Python 3.
Update CHANGELOG