Support form sections in Formotion

Hey,

This pull request add support for Formotion form sections.

By default model fields are put in a single untitled section.
It is now possible to define sections with the `has_formotion_sections` macro and then assign fields to sections.

Changes are tested (formotion_spec.rb) and documented (README.md).

Feel free to comment the coding style so I can update the PR to be on par with yours.

Cheers

/Paul