alabama scraper

Scraper working and tests are passing.

I noticed right as I was finishing up that the site that was on the wiki for Alabama linked to Invitations to Bid (ITBs) rather than Requests for Proposals (RFPs). After poking around the web a bit, I got the impression that the process for bidding on an ITB would be different than for an RFP and thought that it might make the data even richer and more valuable to distinguish between the two.

To that extent, I named the scraper `itbs.coffee` and added a Boolean field `is_itb` to each JSON object returned, which is `true` if the solicitation is an ITB and not an RFP. I also added the `is_itb` field to the schema doc. 

Lemme know if that seems like overkill or if we're not as concerned with ITBs than RFPs. In either case, I'm happy to write another whosie whatsit for Alabama's proper RFPs.