[WIP] Support for generic REST client

[jtopjian]

This commit adds support for a generic REST actions. These actions allow direct REST communication while still leveraging some of the core Gophercloud client features.

For #56

[jtopjian]

@j-griffith As discussed a few weeks back, here's a first attempt at a "dummy" client. It's able to leverage existing Provider and Service clients, so things like authentication and core HTTP calls are automatically handled. Technically low-level calls are already possible in Gophercloud, but there's a good amount of work involved, especially if you're making multiple calls, so I tried to strike a balance between the core calls and the per-package functionality.

If you can, at your convenience, let me know what you think or if you see any issues.

[jtopjian]

Just noticed I can easily add support for URL/query params, too. I'll loop back to this later today.

[j-griffith]

Awesome! I'll try and look at it ASAP

[jtopjian]

@j-griffith All sounds good. I'm not in a rush to merge this, so definitely take your time. I'd rather wait a while to hear feedback from yourself or anyone else who notices this PR open :)