Enable doctests for `python setup.py test`

This is a continuation of #95 ... I didn't realise merging those commits manually into master would close that github issue. Copy of issue description in one of the comments there ...

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I fixed most doctests ... what remains to be done is to configure the doctest runner to skip the following files:
* `gammapy/data/poisson_stats_image/fit_sherpa.py`
* `gammapy/data/poisson_stats_image/simulate_data.py`
* `gammapy/docs/background/estimate_significance_image.py`
* `gammapy/docs/tutorials/image/sherpa_fit_image.py`
* `docs//image/colormap_example.py`

The first four contain imports or non-functioning code and should not be executed.
The last one causes the tests to hang because a matplotlib figure pops up and has to be closed by hand.

I don't know how I can configure the test runner to skip these files entirely?

I currently have this in my `setup.cfg`:
```
[pytest]
minversion = 2.2
norecursedirs = build docs/_build
doctest_plus = enabled
```
I tried putting `__doctest_skip__ = ['*']` at the top of these files or adding their filenames `norecuredirs` list in `setup.cfg` ... but this doesn't work.