Sync package json (closes #1)

Resolves #1 to keep the manifest file up-to-date with the package.json

This implementation uses the postProcessTree hook to manipulate a user's manifest.webapp file with values from their package.json at build time. postProcessTree gives you access to the current broccoli trees as they are going to be built.  With this, we can target variables from the manifest.webapp to be replaced and merge any changes into the build tree.