Switch to pull requests for all mirage/ commits

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

As we stabilise the trees for OSCON, would everyone with developer access to 
the Mirage organisation please switch to issuing pull requests for changes?  
You can merge them yourself if you're absolutely sure that the fix is correct, 
but otherwise leave them open for me or Dave to look at.

The pull request model is good in normal development anyway, as it sends out an 
email to the rest of the committers.  Thomas and I do this in opam-repository 
and don't directly push there, for example.

-anil