Add support for building candidates

[hvr]

this is definitely low-priority, but it'd be interesting to be able to upload a candidate to Hackage, and have the builder test it.

[maoe]

It would be really great if Hackage and the matrix builder supported the workflow like "Publish this candidate if it builds fine on all supported range of GHCs. Otherwise notify me".

[gwils]

I would find this very useful.

Edit: To elabourate: I, for one, find the permanence of a hackage upload quite daunting. Of course I run travis on my projects, and test locally before doing anything like a hackage upload.
At the last stage of the process, this extra assurance that things look good on hackage's end too would help put my mind at ease.

[hvr]

A first pre-req has been addressed by the proof-of-concept at http://cand.hackage.haskell.org/ which has already exposed a few minor shortcomings that need to be addressed before this can be pushed further.