Revive alternate context drawing...

For curves and points only now.
Add a Revit_2015 context...

For alternate context visualization.
Add PolyCurve to revit viz manager.
Fix subtree caching

Subtrees that were deleted and added back as a modified subtree should
be added back to the cache.
Merge pull request #1691 from junmendoza/CottonCandyJun

Fix subtree caching - fix to go in CottonCandy branch
Merge branch 'master' of https://github.com/DynamoDS/Dynamo into CottonCandy