Finish change to Click in gammapy.scripts

[cdeil]

This PR finishes the change in gammapy.scripts to use Click that was started in #1235 .

It adds an extensive docs page for gammapy.scripts, including reference documentation for the existing commands using https://pypi.python.org/pypi/sphinx-click . I've added a lot of information on the docs page how it works, that is mostly targeted at contributors, not end users. I think this is OK, since at the top there is a big fat warning that the CLI doesn't really offer much useful functionality now for end users. I'm hoping that this information will be useful to someone or a small group of people to take on the challenge to develop the Gammapy high-level interface in 2018, and then once something nice is in place, the documentation will very likely be reorganised into parts that are useful for users and technical info for developers.

This PR leaves gammapy as the only command line tool, removing a few others like gammapy-image-model-sherpa or gammapy-data-select. I hesitated to remove the data select to do run selection based on sky location, time or other parameters, but in the end did remove it because it probaby should be re-written in a better way, e.g. using astropy-regions and taking ds9 region strings for more flexible / powerful / simple sky region selection, i.e. the CLI interface and implementation would change. Also, it's still in the git version history if someone wants to have a look at the old implementation as an example or starting point to inform a new better one.