Preference to [bs-form-group], option to suppress warnings & unique error containers

Improvements

1. Giving preference to [bs-form-group] selector over .form-group to support other template design #29
2. Uniquely searching the error container based on the id attribute of the input element to be able to show errors for every input element grouped in a single form-group element
3. Converted two error messages to warnings to avoid noise in Google analytics
4. Added an option to suppress those warnings

Code diff: v1.0.6...v1.0.7