Fixes Umbraco packager

Fixes #117

The fix applied in #100 had been reverted with updating the NuGet reference for the UmbracoCms.Core assemblies (in changeset 61d0b671182ba52cff86147b9d3f631179f34190). This patch reverts the revert :cyclone:

Updated the `Gruntfile.js` to only copy the `Archetype.dll`, rather than all `*.dll` files.  Wondering whether we should allow the `*.pdb` for pre-release builds?