Add audio modem integration for transaction sending & receiving

This allows air-gapped transfer of transactions between a watching-only online wallet and an offline wallet, holding the private keys.

The two additional icons are:

 - http://www.flaticon.com/free-icon/speaker-outline_54951,
made by <a href="http://catalinfertu.com">Catalin Fertu</a> 
from <a href="http://www.flaticon.com">www.flaticon.com</a> is licensed under 
<a href="http://creativecommons.org/licenses/by/3.0/" title="Creative Commons BY 3.0">CC BY 3.0</a>.

- http://www.flaticon.com/free-icon/mic_10032,
made by <a href="http://www.elegantthemes.com">Elegant Themes</a> 
from <a href="http://www.flaticon.com">www.flaticon.com</a> is licensed under 
<a href="http://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.