Cleaned up documentation related to installation and Contributing

Since I just started working with the Python source, I went through the "contributing" guide.  Along the way I found some stuff that didn't work because it looked like some things had moved (both within the source tree, and out on 'the web').  I updated the Contributing Guide to reflect the steps I had to go through to get a working environment and I updated the installation guide to account for the github path change from softlayer-api-python-client to softlayer-python.

Along the way I fixed minor spelling errors and changed the grammar to suit me.