Add "Change affecting skill" context menu, resolves #101

See #101 for progress and discussion of these changes

* Includes two changes to EOS
    * Modifiers are added to modifier list regardless if they modify any attributes at the time. A new `used` parameter is introduced to replace the functionality to marking a modifier as modifying something.
    * Characters load all skills now, and all skills are saved in the database (previous mechanism was that only skills that were modified were added to database as needed). This is done in a way which is backwards compatible with current pyfa installations, and any addition of new skills to the game will also be covered by this change
* Changes the item view to only show modifiers that are `used` (see EOS change)
* Creates context menu that allows us to change a skill that affects a module / ship.
* Adds custom icons to represent skill level

Screenshot:
![skillsaffectors](https://cloud.githubusercontent.com/assets/3904767/3120314/b0d34e52-e74f-11e3-986a-6b99f502bd68.png)