Initial support of relay domains.

see #491
Fixes.

see #491
Support added to relay domain aliases.

I've also refactored some code.
see #491
Better error validation.

see #491
wip: relay domains support

* Unit tests added (limits application)
* Added support for limits
* Big changes were required
* Using standard HTTP codes to identify errors (json)
* Custom exceptions have been reworked (not finished yet)

see #491
ajax_simple_response removed.

see #491
Cleaning exceptions.

see #491
Don't raise ModoboaException directly.

see #491
Adding support for import/export of objects.

see #491
wip: relay domains.

Fixed various issues about:
* Permissions
* Limits

Unit tests were also added.

see #491
wip: relay domains support.

* Integration with the amavis frontend
* Per relay domain policies
* One policy shared between a relay domain and its aliases
* One 'users' record per object
* Extra: better event handlers management

see #491
Properly reload callbacks when an extension is enabled.
wip: relay domains.

* Documentation update
* Few fixes

see #491
wip: relay domains.

Documentation update.

see #491
Properly display sieve filter errors.
It seems I pushed the wrong version :p
Replace old occurences of 'admin_user' by 'core_user'.
Merge branch 'master' into features/relay_domains
Execute all unit tests.
Adding Travis support.