Add support for mapping transform

Unit and integration tests included.  Closes #823.
Fixing build: unit test json files weren't set to Copy Always
Fix multiple transforms test
Merge branch 'feature/mapping-transform' of github.com:elasticsearch/elasticsearch-net into feature/mapping-transform

Conflicts:
	src/Tests/Nest.Tests.Integration/Nest.Tests.Integration.csproj
	src/Tests/Nest.Tests.Unit/Nest.Tests.Unit.csproj