XEP-0191 (blocking)

Adding implementation of XEP-0191 (blocking) based on implementation of
XEP-0016 (privacy)

This extension can do :
- Block jid
- Unblock jid
- Unblock all
- Get blocking list
Code consistency with the rest of project

Replacing NSString by XMPPJID
Merge pull request #289 from phetsana/XEP-0191

XEP-0191: Blocking Command