Add Django 1.5 Custom User model support

Provides support for Django 1.5's custom User models while giving a
fallback for earlier versions of Django
Merge pull request #16 from huxley/custom-user

Add Django 1.5 Custom User model support
Also mention User model fix by @huxley in CHANGES.rst
Moved Django compatibility functions to utils.compat module
Add get_user_model_name() to prevent circular imports

Instead of linking to the User model by class, use the "auth.User"
string so `get_model()` won't be called while `fluent_pages.models` is
still loading. That causes circular imports, which happend at:

* `import fluent_pages.models`
* running `runtests.py`
Bump version of django-wysiwyg_ to 0.5.1 because it fixes WYSIWYG editor
integration.

The `{% wysiwyg_setup %}` tag was broken for CKEditor, Redactor and TinyMCE.
Move template tag parsing to separate django-tag-parser package.

The code is shared between multiple django-fluent-* apps,
and shouldn't be in this package at all.
Code cleanup in tests/utils.py
Updated setup.py trove classifiers
Include django-tag-parser in example+RTD requirements.txt
Include django-fluent-contents in RTD requirements.txt

The `docs/api/pagetypes.fluentpage.admin.rst` can't compile because this
package is missing.
Add Travis-CI build configuration