Fix inactive bug & performance enhancements

I've made some changes to LEColorPicker for my purposes and to fix some bugs
- Removed `UIApplicationStateInactive` from isActive requirement (#6)
- Fixed LEColorPicker worker queue becoming permanently suspended when coming back from being inactive (since there is no complementary state notification for `UIApplicationWillResignActiveNotification` and since `dispatch_suspend` and `dispatch_resume` nest inside themselves)
- Added shared instance of LEColorPicker for class method
- Added specific target queue assignment for worker queue
- Using a shared context for GL since all work is done on a serial queue

Feel free to take them, modify them, or whatever. The bug fixes should for sure be useful for someone. I'm currently using these in production without issue with a ton of images in a collection view and this is performing nicely.