Point to user-configable swagger spec location

[jisantuc]

Overview

This PR makes it so users can point to a specific swagger spec if they need to, otherwise it just points
to master on the raster-foundry/raster-foundry-api-spec repo. Once we have a release of that, it
can point to a specific release.

Testing

• add a print to either the load_url or load_file branch of the api client initialization
• start an ipython session with no special environment variables
• create a client
• verify that your print either did or didn't happen
• call .projects on the client you created and verify it works
• start an ipython session with RF_API_SPEC_PATH set to rasterfoundry/spec.yml
• create a client
• verify that your print either did or didn't happen
• call .projects on the client you created and verify it works

Closes #48