tsd dependency support

[dsvensson]

Support for declaring a list of tsd files that should be pulled in to build, similar to how the bower gradle plugin is configured to install dependencies:

https://github.com/craigburke/bower-installer-gradle

https://github.com/DefinitelyTyped/tsd

Same as #20, play nice with gradle node plugin, would be nice, to get tsd via npm automatically. But at least, as it is now, with a configurable exec path.