Add support for dub

http://code.dlang.org is becoming the de facto standard as a D package repository, along with it's associated build tool ````dub````. Adding this package.json file and then publishing to the code.dlang repo* will allow people to express tango as a dependency for their project and have ````dub```` deal with downloading from this github repo and building.

There's a lot of quality code in tango, it seems sensible that it should be available through ````dub````.

*I will do this if/when this pull is merged.