Move all logic from __init__.py to base.
Allow add domain aliases.

This implies some minor refactor of domains app code.
Add tests for domain app.
Add mission apache license header on main domain modules.
Move taiga.base.domains to taiga.domains
Fix domains module imports due to previous module relocation.
Fix domains module path on settings.
Fix typo on function alias on domains/__init__.py file.
Merge pull request #32 from taigaio/domains-alias

Domains alias