Update Windows install documentation

[khaeru]

@mmowers reports:

1. When running python setup.py install in py-gdx/, I had an error that said Setup script exited with error: Microsoft Visual Studio C++ 10.0 is required during the installation of pandas. I solved by just installing numpy and pandas directly with conda install numpy and conda install pandas, and then running python setup.py install again.
2. I had to make sure gams was added to PATH. I reinstalled and followed instructions here to make sure gams was added to PATH: http://www.gams.com/latest/docs/userguides/userguide/_u_g__w_i_n__i_n_s_t_a_l_l.html#CLI

Update the documentation to make these clear.