Refactor notifier

Lots of refactoring - but still not enough:
- ENV is stubed by default, as it should be (without it, refactoring Notifier isn't possible)
- Notifier specs are temporarily uglier, but they don't rely on ENV or actual Notifiers
- Refactored the tmux notifier, because it was unmaintainable and caused issues with the above
- drop support for tmux < 1.7 (2 years old), because it's so worth it
- make tmux strict about states on -> off and back, to prevent hiding Notifier bugs
- the ENV mocking/stubbing is horrible and will be fixed, but this PR is too big anyway
Perceived complexity for available? is too high. [8/7]