add conda installation instructions

[orbeckst]

Once PR MDAnalysis/mdanalysis#751 is merged, which makes MDAnalysis available through conda MDAnalysis/mdanalysis#608 , we need to document this

• add to Quickstart Installation
• blog post

Also update the tutorial (MDAnalysis/MDAnalysisTutorial#7).

[orbeckst]

Should we say in the post that the conda builds are only serial and that for OpenMP builds you should also build locally?

[kain88-de]

I thought it sounded so negative when we say that in the end of the post after mentioning osx. I mentioned it on the quick install quide though.

The thing is how many people are actually using the openmp backend? I don't want to scare people of the conda build for something they don't even use.

[orbeckst]

Ok, if it's in quick install then it should be fine. Just should be somewhere.