Adding a blank __init__ for git submodule usage

I need the CRC32 module for a separate pebble project - without the
__init__ I can't import libpebble.stm32_crc (or libpebble.pebble for
that matter.)
Pebble autodetection for OS X

On POSIX systems (specifically OSX) this will attempt to auto-detect
the Pebble device node. If multiple nodes are present it will try to
use the most recently used Pebble.

I'm not sure what the Linux naming convention is for bluetooth devices
- the autodetection routine is simple to modify once I have examples.

Windows will require some work with device enumeration since COM ports
are not named.
Error and logging changes

Removes all "print" commands that are not in main; replaces
Exception(message) with PebbleException(id, message); adds a few debug
log statements.
Add requirements.txt

PySerial 2.6 appears to be the current latest; might want to relax the
minor version requirement for older systems as testing continues. All
other dependencies are part of the standard library.