Reuse our BaseForm class within tests and fix uncovered issues

[kbayliss]

Resolves https://github.com/torchbox/tbxforms/issues/44

• Re-uses our BaseForm within tests as opposed to testing arbitrary forms
• Allows changes to a form's helper after instantiation
• Fixes overriding the label size and tag for Field.select()
• Update README to be clearer
• Renames BaseForm to TbxFormsMixin to more accurately convey what it is

[kbayliss]

This work has been merged as part of https://github.com/torchbox/tbxforms/pull/52 (it was easier to do within an integration branch due to so many related changes)