Expose methods other than GET via restful API

[demianbrecht]

Currently, Salesforce.restful only allows for get API calls. There should be a mechanism that allows a user to use other HTTP methods as well.

Reported by @babakh.

[hello-josh]

The bigger problem is that some methods use requests on their own instead of using _call_salesforce like the rest of the codebase. #54 also has the same issue.

[demianbrecht]

@TRII: I might be missing something, but #54 uses self.request which is an instance of requests.Session, not requests directly (unless that's what you're referring to). Is there elsewhere that I'm missing where requests is used directly? The response handling should be generalized between #54 and Salesforce.restful though.

There is another issue in that SFType and Salesforce don't share sessions, but that's captured in #91.

[hello-josh]

I was referring more to mixed use of the requests.Session instance vs the internal _call_salesforce wrapper. For consistency's sake, everything should use _call_salesforce, and _call_salesforce should use the requests.Session. If not, there is no reason for _call_salesforce to exist.