Separate example, requirements.txt, CAD, SNR-adjusted RSSI, formatting

[calebj]

This is just a bunch of changes I put in and forgot to commit along the way, so here's the ultimate list:

• Move the example off to its own file
• Format to make pep8 linter happy and remove trailing whitespace
• Integrate the datasheet formulas for SNR-adjusted RSSI
• Add channel_active method
• Correct imp_header kwarg to implicit_header in set_modem_config_custom() signature
• Protect SPI access with a lock so the callback thread doesn't fight over it with regular calls
• Change string helper methods to static methods and simplify them
• Remove a stray print statement
• Add a requirements.txt for convenience

Plus a TODO that should probably be taken out of the file; it's just a list of things I'd personally like to see implemented sometime.