Created a "Common" project for shared classes; AssemblyInfo tweaks.

Working on Pub integration, which will reuse this class (and maybe others); but it previously lived in the DartAnalysis project; and I'd like to keep Pub integration separate without a dependency.
After #89 is merged, this line will need to be updated to modify the resource strings in **VSPackage.resx** instead of the string literals used on this line.