Support custom validator
Update docs and bump version -> 0.5.2-dev
Py3 fix: TypeError raised if field is not an object
oopsie daisy
Add unittest2 to dev-requirements
Fix datetime validation
Add from_rfc utility function and add tests for datetime validation
Validate Date, Time, and Function fields
Add validation tests

also add Method field validation
Update changelog
Bump version -> 0.5.2
Default Integer field to 0 instead of 0.0

Fixes #13
Remove unused import
PEP8
Merge branch 'pr/15' into dev
Update version -> 0.5.3-dev and update changelog
Optional context argument can be used by Function and Method fields
Function field initialization fails early if argument is not callable.
Method field raises error if method is missing or uncallable
Raise error if no context is available

Also, update docs