#4 Add encryption

[jaumedejuan]

- Possibility to encrypt conversations.
- This will generate problems if using the Avetana stack:

Avetana limitations, extracted from:
http://www.avetana-gmbh.de/avetana-gmbh/produkte/Readme.xml

Not all Bluetooth-Stacks support all the features that JSR-82 specifies.
Here is a list of unsupported Features of JSR-82 depending on the
Operating Systems (M for MacOS X, W for Windows (Widcomm), L for
Linux)
- Searching for services that are not in the public browse group (w)
- Encryption, Authentication (l)
- Encrypting existing connections (m, w)