Support a separate harvest env for dev.
Provide a mechanism for working locally

The app previously couldn't be run without a public IP b/c it would hit
github on most requests. This override flag will allow you to work 
locally so long as a user with a given name does exist in the DB. They
still need a github record associated with them for much at all to work
but if it's set up once it'll be fine.

To pull it off, just do something like:

LOCAL_OVERRIDE_AS="Justin Searls" rails s
Force the user to send invoices from #show action

Instead of firing off invoices on first-press, generate the invoice 
first and allow them to be sent to harvest from the Show action page.

This will let us summarize ready_to_invoice at a glance before anything
goes to harvest