Refactored and other useful things

Once again thanks for the useful library and I hope you like the changes I've made to it. Considering there's not anything else that does nice transitions it's worth building on.

Quite a few things changed but most notably the following:
- Refactored to remove HUMP.timer from venus.lua
- Refactored venus.registerEvents with some inspiration from HUMP.gamestate.regsiterEvents
- Ability to set the duration of the transition
- Can register a HUMP.timer object. Will user 'Timer' if one isn't provided.
- Changed how the stateless concept was handled by mocking a dummy state instead.
- Added all of love's callbacks to all_callbacks
- Removed global callbacks. I figured it was better served by using loves default callbacks in main.lua. Feel free to disagree.
- Refactored the code for each effect. There was a lot of checking of existence. I've made venus.switch assert for a few things. If someone manually calls one of the functions out of order so be it. It'll break and they'll know it did.