Add SqlPackage to the repository.

This is a handy tool included with SSDT for packaging databases.
Possibly it should be in a different location, but this is where the
other SSDT binaries have gone (on the database-project branch, for
building the database project on TeamCity), and I'm not keen to
duplicate binary files in the repository.
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.
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.
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.