Add MapMaker run method

[AtreyeeS]

This supersedes PR #1561 to solve merge conflicts

[cdeil]

Please add a test or change the existing test to exercise run for one example (could have one or two runs).

[AtreyeeS]

Hi! Added a test.

But I don't really see the added advantage of having a MapMaker.run(). It simply returns a dictionary right? The user could have achieved this quite simply.

Or maybe I am missing the point @mackaiver was trying to make!

[cdeil]

@AtreyeeS - The whole MapMaker is a convenience high-level interface, nothing there is really needed.

Adding run taking an obslist is equivalent to
http://docs.gammapy.org/dev/api/gammapy.spectrum.SpectrumExtraction.html#gammapy.spectrum.SpectrumExtraction.run

A discussion on API patterns in Gammapy is coming soon, but for now, let's just continue this pattern here with the MapMaker that basically one does something like:

results = MapMaker(config).run(observations)

[cdeil]

@registerrier - I'm merging this in now, to avoid merge conflicts and so that we can move ahead with more work on MapMaker tomorrow.