swarmchat: initialize empty git repository with initial commit

This patch offers an API to create conversation. A conversation
is represented by a git repository.

The repository will contains the account certificate in /admins,
its CRLs in /CRLs and the device key in /devices. Moreover,
the first signed commit's hash will be the conversation's id

Change-Id: Iac7ff7f6002185a5bb0f51d4c10df1afe9e059b4