Add async API

For longer lines and larger patterns, an async API is needed.

This will be in the 2.0 release, which won't be backwards-compatible since the synchronous methods were renamed to end with `Sync`.

- [x] Use `mrblen` instead of `mblen` for thread safety
- [x] Add caching to async API
- [x] Test on windows
- [x] :memo: Doc new methods in README
- [ ] Add CI for node `0.10` and `0.11`