Don't bind to all .field elements, only the one closest to us.

If you have a list of TinyMCE fields, then each the whole list will be
a .field, as well as the RichText field in question. Without this,
$field binds to both, meaning the format select boxes affect all fields.
Allow text_format to be overridden by plone.app.textfield

It already knows the mime-type of the field, so no need to jump through
hoops working it out.
Merge pull request #83 from lentinj/master

Fixes to improve plone.app.textfield in Lists