Add support for TypeScriptOutDir. #3

The Visual Studio TypeScript compiler can optionally compile TypeScript files (.d.ts, .js.map, .js) to a directory other than the originating typescript file. When the user sets that up, this build task should look for the files there.

For first pass, assume that TypeScriptOutDir is the same for all configurations. Adding configuration detection/support is likely to prove difficult.

Make sure this works with .ts files that are in a folder rather than in the root of the project.