Fixed name in exception message.
Revert "Fixed name in exception message."

Validators shouldn't have access to name of attribute, under which the
field is - this implies many problems.

This reverts commit a0c6299b080078136e90f78c37a0bb263980c3fe.
Removed field name from validation errors.
Fixed validation of list fields (fixes #47).
Improved technical docs.

* Removed support for `PEP257` - it require sometime some obvious doc strings.
* Removed obvious doc strings.
* Improved some docs.
Refactor.