Add psake as a git submodule.

If you wish to build from the command-line, you will need to run
`git submodule update --init` first.
First pass at a simple build script.
Run builds in parallel.
Improve the build script a bit.

A few small improvements:
 - Add a handy run-build.ps1 script that will import and invoke psake
for you.
 - Inject the value of the root directory property
 - Remove the helpers.ps1 script, as this is no longer required.
Merge branch 'build' into staging

Add a couple of psake scripts to start off the CI build bits.