Pub integration

This change runs "pub get" when a project is created; which will fetch any packages (similar to NuGet Package Restore).

This will fix any errors/warnings in default templates caused by references to pub packages.
We need to get out the output pane here (I think pub output is valid in the normal output pane, not just DartVS Diagnostics). Not sure how best to do this, especially as these classes don't seem to resolve MEF imports.