Add Mumbai with Kolkata's details

Fixes #13.

I did some digging. [Moment Timezone](http://momentjs.com/timezone/) uses [this timezone database](http://www.iana.org/time-zones) and according to [this](http://grokbase.com/t/perl/datetime/128heb2gan/is-there-timezone-data-for-any-indian-cities-such-as-mumbai-dehli-c), the only Indian city is `Asia/Kolkata`. It's a shame some cities aren't included but I have added Mumbai using Kolkata's reference.

```javascript
"mumbai": ["Mumbai", "Asia/Kolkata"],
```