Implemented -rbt_stateSignal
Send initial value on subscription to -rbt_stateSignal

- Add libextobjc as a submodule
- Fix some documentation mistakes
- Publicized RBTCentralManager.h and ReactiveBLE.h
- Set up a bare Mac app linked to ReactiveBLE for manual testing
Don't need a prefix
Fix signal name
Follow GitHub/RAC style guidelines
Such style, wow.
Remove redundant -takeUntil:
Revert "Remove redundant -takeUntil:"

This reverts commit bfdf757fd222c4cd321cc3077dccb3827224f4ce.
Merge pull request #3 from indragiek/state-signal

Implemented -rbt_stateSignal