enhanced README + issue #1

Hello, I made a few adjustments to your README trying to make it more clear.

I got a problem using your setup.py:
BSON dependency isn't necessary, and it is causing this error:
http://stackoverflow.com/questions/13123155/is-py3compat-ise-used-in-pymongo
If you got this error, please pip uninstall bson and py3compat.

Also, there was a missing comma in setup.py. So, this PR closes #1