Large installation documentation update.

This is a complete rewrite of the install documentation based primarily on the feedback from SIPWork.

I have made Anaconda the default install path, relegating the other options for installing scientific python to the Advanced section. I have also tried to make the documentation as simple as possible and generally make it easy to follow.

I have also modified this documentation to use the testing changes in #1180.

There is a built version of the docs here: http://sunpy.cadair.com/docs/install_doc_update/. 
What do we use skimage for? or mock?  - maybe a comment on the file?