mySQL fast import

Uses a staging table for mappings which is truncated before a file import.
Uses LOAD LOCAL INFILE to fill this table, followed by an INSERT.
This is mySQL-specific, but it's about 1000x faster than the Ruby we had.
Import more than one via file mask
Make the fast import deal with updates
Import/update suggested and archive URLs if present
Merge pull request #7 from alphagov/seed-mappings

Seed redirector mappings using mySQL `LOAD INFILE`