Upgraded to Pebble SDK 2.0.2.

- added pypng resource
- modified pebble-arm-toolchain resource to the precompiled one given by Pebble
- removed the dependency from virtualenv
Removed unutilized dipendencies
Fixed Pillow compile error.
Revert to building pebble-arm-toolchain from sources.
Revert "Revert to building pebble-arm-toolchain from sources."

This reverts commit 14b12b004d052ad611265424f25efa0fc302ec09.
Added dependency macos >= mountain_lion
pebble-sdk: add 2.0.2 bottle.