Insertion query is fixed
Added unique constraints to the tables.

This should fix issues where multiple sources of data are attempting to be
loaded. This errors on a file by file basis so may cause issue when
daily and monthly data are added together...
Interface to DB queries have been written.

This is a major upgrade which Fixes #16 and Fixes #22.
can now use the "query_offer" method to automatically query from
the DB tables and return results as OfferFrames.

No need to worry about where the Data is coming from.
It should "just" work. I've specified that a date range is required or it
will try to query everything which is about 2GB of data per master table.
Not really sustainable.

I'm quite excited by this. I still need to write some documentation and
there is some very basic type checking.