Set the default value in a validator after the required check

Fixes a bug where a non-required parameter set to an empty string was not set to null in the cleaned array.
Merge pull request #7 from wave-framework/validator-custom-violation-messages

Small fix in the TypeConstraint file
Make the XMLResponse send XML (fancy that…)
Parenthesis error in float validator
Fixed phpdoc for generate models
Merge branch 'master' of git://github.com/wave-framework/wave
Fixed query for null related columns.
Moved exception logging into the try-catch

No more 'Exception thrown without stack frame at line 0'
Namespace duh.
Removed null check for one-to-many (unnecessary)
Allowing custom violation messages on exists & unique constraints

- can be used via the 'message' property
Merge branch 'master' of github.com:wave-framework/wave into validator-custom-violation-messages