Merge tag '0.3.0'
Differentiate the exceptions raised by `one()`

Fixes #15
Use native storage for list, map, and bool
Adding support for multiple validation checks
Support easy non-null checks
Fixing serialization of datetime and date types
Updating changelog
Fixing tests for python 3
Adding docs for field validation checks
Simplify index-selection code
Moar tests, moar code coverage
Bump version: 0.3.0 → 0.4.0
Merge tag '0.4.0'
Move pep8 configuration to setup.cfg

pep8 documentation says that .pep8 files are deprecated, but it will
automatically read settings from a [pep8] section in setup.cfg. I've
moved the .pep8.ini content to setup.cfg, which means that the pep8
command won't need the --config parameter anymore.