merged Field and FieldGroup classes
Returning a non-instantiable type as model is not funny.

If validation fails, that should be done at instance level, not class level.
Error shall never pass silently.
Rename unit-tests' internal functions.

* Unittests are not integration tests, they shall not not be numbered and
  perform various steps operations;
* do not use transact_* pattern for internal functions using @transact, as
  usual.
Creating assert_model_exists, and remove similar functions all around unittests.
Rename and improve unittests for models.

* Merge TestComposingFields and TestNextGenFields into a single TestField
  class.
* Create TestStep for stuff to test steps, and not field.
Sorting imports in test_models.

Note: globaleaks.models.models is a list.
      globaleaks.models is a module. This naming conflict shall be resolved as soon
      as possible, exploiting __all__ may be.
Replace assertTrue with assertIs[Not]None whenever possible.

Properly use assertion in test_models unittests.
REMOVE FUCKING TEST ENUMERATION FROM MY FUCKING HANDLERS
Test admin handler: create a unique class-attribute `sample_request.

Why duplicating code everywhere when we do have this cool thing called ENVIRONMENTS?
Test Admin Handler: spotting unicode bug in put.

Apparently the server accepts non-unicode stuff and throws out str(), not
unicode(). This must be fixed.
Check globaleaks.tests.handlers.test_admin.TestAdminFieldsInstance.test_put for
more informations.
Create tests.handlers.admin package.

This file rename has become necessary in order to:
- tidy up unittests
- have a __future__.unicode_literals import and start some good practices in unicode
Sort imports in tests.handlers.admin.test_field.
Cleanup in tests.helpers.

* Use single-quoted string conforming to the rest of the file [PEP8]
* Use format instead of % operator
* Remove bad and ugly try-catchall statements.
  (cuz @vecna just wanna catch'em all)
  http://www.youtube.com/watch?v=SVp-zIONsrs
Top description shall be documentation (in admin field handler).
Testing with unicode_literal in handlers/admin/test_field.

The failure is still there, comes from the field option being None instead of
empty dictionary.
Fixing and testing admin/field.

* Remove transact_ prefix from function using @transact, as I don't see the same
  pattern in other handlers.
* For some reason, test_post had no real assertions. Fix this.
* s/Fields/Field/, following the convention
* 'option' shall be an empty dictionary in the serialized data.
Fixing get for admin field instance, and adding proper tests.
Removing unused imports.
Fixing and testing delete for admin field instance handler.
Cleaning up BaseHandler.

* Sort imports in basehandler.
* Use str.endswith instead of slicing.