conference: first implementation

This commit adds conferences management

- create conference by joining two ongoing calls/conferences
- create conference by calling a new peer and joining him to the ongoing
call/conference
- hanging up conferences

This commit relies on CallModel and RecenModel selection models
interactions to update call view and Smartlist row selected.

Change-Id: Ib9f6b9b870e66c8ccfb5115ad6fdb592c625eb58
Tuleap: #114