Caching of javascript files to improve performance

It was annoying how each time I modified anything, even an HTML file, each .ts would get queued for recompilation by Meteor. It should only recompile .ts files that I just modified... otherwise this defeats the point of hot code pushes.

I added caching of generated javascript, but there's a caveat. **Changes to .d.ts files do not trigger cache updates.** It's up to orefalo whether to allow this oversight, as it means that type changes in a .d.ts file won't create errors until the depending .ts files are modified. This issue could be fixed when Meteor comes out with a way to get all files to compile at once, but until then, the benefits of fast development outweigh it for me.