Add a database backup task

We probably should have had this before going live, but never mind... This pull request adds a couple of scripts to export the database as a BACPAC (a deployment artefact encompassing both schema and data) using the SqlPackage utility from SSDT. The resulting BACPAC can be right-click deployed to any SQL Server 2012 instance via SSMS.

I've added [an associated TeamCity build configuration](http://build.dddeastanglia.com/viewType.html?buildTypeId=bt7) which runs daily at 00.05. The configuration publishes the created BACPAC as an artefact, so we can download any backup from TeamCity.

I've deliberately sent this pull request to the production branch, because it has no effect on the product itself and so can safely skip the staging process. I have tested it against my own fork and watched the build fail and finally succeed. Given the nature of the task, I would like this to go live ASAP.