Remove as_string from Arbitrary and Fixed
Basic field-level validation and error storage

And fix invoke test command
Small validation refactor
Add param for binding extra data
Add Method field
Raise KeyError if validating a nonexistent field
Add Flask example
Allow relative urls if relative is set to True
Add Function field

Also, change version to 0.2.0-dev
Remove unused functions

And make tests more backwards-compatible with py26
Bump version 0.2.0-dev->0.2.0