Add backup.ps1 script.

Heavily based on the existing build.ps1 script. Call SqlPackage from the
build\targets\SSDT directory, and output a BACPAC to a backups directory
in the repository root.
Clean up existing backups; create the destination.

Remove any existing backups: these can be published on TeamCity as
artefacts, or simply copied to a new destination if run manually.
Discard the output of new-item so it doesn't clutter up the console.

Add a few comments to the script to describe what it's doing.
Add a run-backup.ps1 helper script.

The script invokes psake on the backup.ps1 script; you probably want
this one rather than backup.ps1. You have to pass in the meat of the
connection string (server, database, username, and password) on the
command line.