Add small fixes for bootstrap 3. Add tests.

Hi,

I use this gem in a current project so I added some fixes to make it work better with Bootstrap 3. I also added tests for everything.

Details:
I removed the example code for inline. Correct me if I'm wrong but this wasn't TWBS 3 markup. Inline form elements in horizontal forms are also not mentioned in the Bootstrap docs so maybe we should remove this? If it should stay, I think the right markup would be to add a row container and add a column class to every form-group inside that row.

Also the date_select won't render 3 fields on one line without extra markup in TWBS 3 so the old screenshot is wrong. We would need to add extra css to make the 3 select inputs float or wrap them with a row and add column classes like for inline inputs.

Updated the height sizing classes to use *input-sm* instead of *small*.

Added the select element styling fix from https://github.com/stouset/twitter_bootstrap_form_for/pull/78

Updated the screenshot.