added a main for JSPM 
added a main for JSPM 

jspm people like me would love it, if we could have our main defined such as this.
that wouls allow us to do:
```
jspm install socket.io-client
```
and it would work without us overriding anything in jspm registry.
nus-fboa2016-si/socket.io-client,Alexandre-io/socket.io-client,socketio/socket.io-client,ayhansalami/socket.io-client,nkzawa/socket.io-client,socketio/socket.io-client,Nibbler999/socket.io-client,socketio/socket.io-client,Automattic/socket.io-client