custom styling of validation messages is not available

[prabhatsharma]

Validation mesages were appearing a bit crazily in our application due to various styles and I realized that help-block class is hard coded for styling of validation messages. We need support for custom styling of validation messages.

I have forked the repository and provided a fix for this. Will be raising a pull request.

[prabhatsharma]

Have added following pull request
#444

[nicklasb]

Closing, the discussion may continue at the PR.