WARNING: API changes

This patch makes code common for IM send/receive message API's
between call and account classes.

Multi-part messages is supported for :
- SIPAccount
- SIPCall (any SIP like account)

RingAccount supports only Single-part messages yet.

Change-Id: Ic65425cd95f02f89f03dd6ea2a9c4a10bb233859
Tuleap: #157