updated session.py to better match the ShopifyAPI ruby gem and updated t...

...he readme

@costford @maartenvg 
@dtKinger @jonasll 

related to:
https://github.com/Shopify/shopify_api/pull/80

Changes
=======

* Updated session.py to match the functionality in the ruby api gem
* Updated the readme to match the ruby gem
* tested the getting started procedure

Note - This removes the useful "request_token" method. I like the idea of this method and the main reason for removing it right now is to maintain consistency with the ruby gem. I'll likely be adding this method back into both libraries in the future.