Add PlainText field (Static Control) for the Twitter Bootstrap 3 framework.

This functionality allow to place plain text next to a form label within
a form.
For more information, here the Twitter Bootstrap 3 doc:
http://getbootstrap.com/css/#forms-controls-static

For other framework than 'TwitterBootstrap3', it fallback to the
Uneditable field.

Close #377
Rename PlainText class to Plaintext.

Because POSIX filesystems are case sensitive unlike Windows.
Merge pull request #379 from tortuetorche/patch-1

Add Plaintext field (Static Control) for the Twitter Bootstrap 3 framework