Started #30, continued #32 & #12, implemented #18

Features:
- Implemented a huge part of creating new texture & model tasks.

Refactoring
- Removed name argument from Task constructors
- Moved more info into TaskType. TaskType can now convert from and to
Task Classes and tab names.
- Renamed gmm.service.assets to gmm.service.tasks
- Implemented TextureTask form (no conflict handling yet)
- Moved user managment controller methods from AdminController to new
class AdminUserController
- Moved Task Import, Editing and Creating from different places
(TaskController & AssetImporter) into a single new Service class
TaskCreator. TaskCreator class handles everything where TaskForm is
involved consistently.
- Current Task tab is now strongly typed in TaskSession class
- Added jQuery plugin for combined ajax form submit and ajax data POST
requests