Rework API

Per previous discussion, we've:

- Split sailthru.rb into several files, but the bulk of the client API remains in a single file
- Lightened up the dependencies as much as possible and switched from shoulda to minitest
- Renamed Sailthru::SailthruClient to Sailthru::Client and similar renaming for custom exceptions
- Added a global credentials API so the developer does not need to pass the creds to instantiate a Client.
- Removed the deprecated `send` method.  The other deprecation remains because I wasn't clear how to fix the tests.
- Removed the `verify_ssl` toggle.  This was an editorial decision but I don't like APIs that allow the user to disable security.

The core API methods weren't touched except to fix whitespace/indentation issues.

Hope all this looks good.