Team Foundation Source Indexing task add (TfsSourceIndex)

Summary
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I've added a new source indexing task `TfsSourceIndex`. This task indexes pdbs writing the correct  index file to enable source retrieval from a Team Foundation Server source control system. 

Use Case
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Users running TFS and using TFS build server will use the out of the box support for source indexing. However users that want to run a build system out another CI setup like `TeamCity` will find this task useful

Implementation
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The implementation on a pdb indexed by the TFS `Microsoft.TeamFoundation.Build.Workflow.Activities.IndexSources` running on TFS2013 default template.

Testing
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Windows SDK 8.1
Team Foundation Server Client 2012 / 2013
TeamCity 8.06