Fully support test.salesforce.com, and handle windows home location.

A fix for issue #1, implementing the suggestions from @lstrupinskaya. I ran into the same issues myself while using [salesforce-bulk](https://github.com/heroku/salesforce-bulk). So now:

* If there is no HOME environment variable (such as in a Windows environment), join HOMEDRIVE and HOMEPATH together instead.
* Check the sandbox flag for *all* outgoing requests to salesforce.com and properly point to login.salesforce or test.salesforce as appropriate.