Various fixes and enhancements (see details..)

This pull-request takes care of the issues I've come across while trying to run my own Watchfaces as well
as providing a few new enhancements:

- fixes #9   **_Crash on setting timer_**
- fixes #13 **_OSX Build Issue_** -- my fault sorry..
- fixes #14 **_Infinite battery notifications_**
- fixes #15 **_layer_insert_below_sibling causes infinite loop in layer_add_child calls_**
- fixes #16 **_layer_add_child does not remove the layer from it's old parent_**
- fixes #17 **_Missing API: graphics_draw_rect_**
- fixes #18 **_[enhancement] Update pre-built libPebbleSim.a_**

Sorry for messing with the OSX makefile.. I slipped the final build test before pushing.