Adds (mostly) support for Windows and CI

Adds Appveyor CI for Windows (free), and Travis CI for Linux (also free). Also updates our .gyp configuration to compile on Windows.

Nick, if you add me as a collaborator, I can connect my Travis CI and Appveyor accounts directly to your maintained repo. Right now they're connected to mine.

Tests are *disabled* on Windows because not all of them work yet, and it was mostly an afterthought.