Merged "StartConfiguration" changes from "master" branch

This pull request intents to merge ```StartConfiguration``` changes from [another pull request](https://github.com/DynamoDS/Dynamo/pull/2180) into ```Revit 2015``` branch.

Notes to Reviewers
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Hi @ikeough, @pboyer, please take a look. Thanks!

**IMPORANT**: Note that the static method ```RevitDynamoModel.Start``` defaults ```StartConfiguration.Context``` to ```Core.Context.REVIT_2014```, the same as existing code that is already on ```Revit 2015``` branch. Is that intentional or it should be updated to ```Core.Context.REVIT_2015```?
Yes we do. If this is in the 2015 branch, it's a regression.