Parameterizing build script

The meat of this PR is these three changes to the build script:

1. Instead of hardcoding `/vagrant_data`, the script introduces a new variable named `PROJECT_ROOT`. This gets set to the directory containing the build script (`build_site.sh`). This way, this build script can be used in the dev environment as well as in Jenkins.

2. The `TARGET_DIR` is kept within the `PROJECT_ROOT`. Again, this will make it easy for this script to be reused with Jenkins.

3. At the end of the build script, there used to be a command to copy the site `TARGET_DIR` to `/var/www/html/developer.rackspace.com`. We can get rid of this by simply making the `TARGET_DIR` the document root in the dev environment (hence the update to `deploy/dev_web.yml`).