Handle URIs longer than 255 characters

Store the md5 hash of page URIs as well as the original URL as a text field.  Only index the uri_hash field, and query against that instead of the URI.
Merge remote-tracking branch 'upstream/master' into scan

Conflicts:
	data/database.mwb
merge conflicts
md5 is only 32 characters
Handle the possibility of hashes colliding
Fail early
Iterate over possible pages, always return the most recent
use the URIForScan RecordList for getPreviousScan()
Store the raw md5 hash
Don't store a hash field

It was decided that using a partial index would be fast enough, especially with a combined index on the scans_id.
Merge pull request #29 from mfairchild365/scan

Handle URIs longer than 255 characters