Support for multiple users and API keys

This will allow comments to appear from individual developers. If the author is not found in the ASANA_USERS variable, the ASANA_KEY will be used. As described in the updated `README.md`, the ASANA_USERS variable is formatted as:
```json
[{"username":"FirstName LastName", "key":"XXXXXXXX.XXXXXXXXXXXXXXXXXX"}, {"username":"FirstName2 LastName2", "key":"XXXXXXXX.XXXXXXXXXXXXXXXXXX"}]
```

This resolves https://github.com/jamieforrest/github-asana/issues/7 and is based on work by @LarsHassler (https://github.com/LarsHassler/github-asana/commit/1399ea805224f1fbdc09299c1c2e85cb0e94cf15)

I haven't done much with javascript, so let me know if you see any issues with this.