Move to MIT license

Chris mentioned moving to a more liberal license on the [mailing list](https://groups.google.com/forum/#!topic/light-table-discussion/A6q0DXHfuwI) a while back and it's something I'd certainly love to see, so I thought I'd push it ahead (and give people a last chance to voice objections, of course).

I think approval from the @LightTable/committers is the most important thing here, so if I can get that I'll go ahead and merge.

@ibdknox is there anything I should do apart from changing the LICENSE.md file? Let me know if you want any tweaks – I mentioned Kodowa Inc in the copyright but I'm not sure if that's the right thing.