#3 Bluetooth PIN pairing

Add the ability for pairing using Bluetooth PIN, in the following fashion:
* The user explicitly asks to pair with a certain bluetooth client
* swentry provides a PIN string for pairing
* The user enters the string and does the pairing
* Later, when the user tries to authenticate with his bluetooth client, he or she will be automatically granted access.

Fortunately, most of the features are implicitly provided by the protocols, we just need to focus on PIN generation.

[Anonymous]

PIN generation left. The rest is done out of the box for the moment.