Support string quoting in env interpolation.

New option ':q' to interpolation options to double quote a string.
Add ':q' to MSVC library paths, libraries and include paths.

Closes GH-215
Allow custom labels on generic cpp tasks.

Set the environment variable _CUSTOM_LABEL to e.g. "MyAction $(<)" to have that
show up when building a particular set of files.
Fix getting extension of files with dots in dirname.
Add a little unit context when DAG generation fails.
Provide a syntax to force response file generation
Added .NET (C#) example
Added optional TUNDRA_LUA_PATH so that tundra can include additional script files from a user defined directory
Merge pull request #224 from emoon/master

Added .NET (C#) example