setup.py improvements

I improved setup.py et. al. so that I could install pvlib on my systems and virtualenvs. I think that it should work for anybody else too. 

A new user could get up in running with
```
git clone https://github.com/Sandia-Labs/PVLIB_Python.git
cd PVLIB_Python
pip install . (add -e if you want to retain development capabilities)
ipython notebook
```
This would automatically install numpy, scipy, pandas, ipython. It does, however, require pip.