add zenodo doi to readme, add preferred citation(s) to readme and docs

[wholmgren]

@Calama-Consulting and I discussed creating DOIs for pvlib releases. I just hooked up our repo to Zenodo so that it will automatically create DOIs for each release. I'll add the badge to the Readme after we release 0.2.

To test the system, I rereleased the PVSC 2015 repo to trigger the DOI creation and then I added the new badge. It takes a little bit of time for DOIs to propagate, but hopefully the link on that readme will work soon.

We should also discuss how we would want people to cite pvlib in academic papers. See e.g.

http://ipython.org/citing.html

http://matplotlib.org/citing.html

http://deeplearning.net/software/theano/citation.html#citation

My only recommendation for now is to just use the papers in the PVSC 2014 and 2015 proceedings. I don't know how to specifically cite github contributors.

[wholmgren]

Added the v0.2.0 DOI. I will leave this open for a citation discussion.