simple-salesforce updates

Hi @nickcatal,

Thanks again for open sourcing simple-salesforce, it is a handy library.

Here is a laundry list of updates I made in our use of the library, including:

* a bug fix in login.py
* removed the direct `urllib` dependency by relying on `requests` functionality
* added some comments
* added `query_all` as a utility. Unfortunately I haven't tested it on very large result sets in SFDC so it may not be fully correct.
* parameterized the SFDC API version string

I hope that the changes are acceptable and useful. Please let me know if you have any questions or concerns.

Thanks,
Greg