Fix broken Unique

This pull request fixes several problems with the Unique validator.

* InstrumentedAttributes were not working correctly (https://github.com/kvesteri/wtforms-components/issues/13). The test that supposed to test this wasn't working correctly.
* Avoid failing silently if wrong syntax is given, e.g. Unique('name', 'email') instead of Unique(('name', 'email'))
* Add more tests for better test coverage