rewrite installation doc

[wholmgren]

I rewrote much of the installation documentation. I think it's an improvement, but I hope that people with less Python/git experience will be able to make it better. I'll probably merge this tomorrow before many people have a chance to comment, but please still look it over and provide suggestions that we can use in the future.

Here's a link to the new instructions:

http://wholmgren-pvlib-python-new.readthedocs.org/en/installdocs/installation.html

[dacoex]

I suggest:

conda install -c pvlib pvlib
# or, if not using conda, e.g. on WinPython:
pip install pvlib

[alorenzo175]

I think you should also say that once Anaconda is installed, pvlib stills needs to be installed via one of the methods below.

[wholmgren]

Thanks for the comments/suggestions.

What I sometimes do not understand with development mode and the git branching concept where all branches are the same directory but change with checkout command:...

Yeah, this can definitely be confusing until you have a better understanding of git. I would be nice to include links for that sort of thing, but I don't know of one. I don't know if it's possible to explain when and how to use different clones in different conda environments with many branches and multiple git remotes to someone that has never used version control, python, or environments. Help wanted, to say the least.

[dacoex]

This comment was not included:
https://github.com/pvlib/pvlib-python/pull/138/files#r56814196

Donno if it's helpful.

[wholmgren]

I thought that the text that I added to the proceeding paragraph addressed your concern in a more general way. I am open to future changes to this page in new PRs.