Team Foundation Source Indexing task

Added a new task that indexes PBD's so that source can be retrieved from
a Team Foundation Server source control system. Based the implementation
on PDB's already indexed by the TFS
Microsoft.TeamFoundation.Build.Workflow.Activities.IndexSources
Targets includes new TfsSourceIndex task
Updated to query item specs from server. More reliable than local workspace info
Merge remote-tracking branch 'upstream/master'
Allow for pdb's with no source information
Merge pull request #111 from ewilde/master

Team Foundation Source Indexing task add (TfsSourceIndex)