Mistral spec repository

From xylan:

As I mentioned before, Mistral is a small project compared to many other projects, but that doesn't mean we could do anything 'freely'. A good thing recently is, we started to use etherpad to bring new features for discussion before people decide to implement. However, etherpad is so fragile that we may lose the 'history' of feature evolvement process, I'd like to propose we use a specific git repository (*-specs) or something like that to propose, discuss, iterate and track approvals on specifications using Gerrit instead of Launchpad, like other projects, which is very useful for new comers to learn how Mistral evolves, where we are, and where we're going.