Made changes for #2554 requested by @timgraham on Feb 6, 2014.

- Moved ModelForm test of new ``limit_choices_to`` functionality to
  ``./tests/model_forms/*`` instead of keeping them in
  ``./tests/admin_views/*``.
- Added new test in admin_views for ``limit_choices_to`` callable
  functionality for Django Admin.
- Updated the release notes for Django 1.7 to include a note about the
  new behavior for ``limit_choices_to``.
- Added 'versionchanged' notification for ``limit_choices_to`` in the
  documentation for Django 1.7.
- Added explicit link to ``raw_id_fields`` in the changes to the
  ``ForeignKey.limit_choices_to`` documentation.
- Defined ModelForm for tests using more canonical 'fields' instead of
  'exclude'.
- Refactored datetime import statement for model_forms test.
- Phrased changes to documentation in a better way where requested.
- Added missing indentation for 'note' section of proposed changes to
  documentation.
- Simplified inline code comments where requested.
- Removed unused imports.
- Refs #2554.