Allow custom error messages
Refactor: use validated decorator
Remove use of implicit kwargs

Also update docs
Rename NumberField -> Number
Add a Selection Field
Addings docs
Return the value
Add fields.Selection tests
Merge pull request #7 from gisce/fields_selection

Fields selection
Rename Selection -> Select, selection -> choices
Experimental self-nesting
Update version number and add tests
Exclude instead of deleting self-nested field
Update docs
Don't implicitly serialize a collection
Update docs
Update examples
Allow nesting of single values

by setting "only" param to a field name
Update docs
Py33 fix