added support for calling loginWithLinkedin with a callback without opti...

I've added support to call loginWithLinkedin with a callback but without options as it is for the other core packages (see for example https://github.com/meteor/meteor/blob/devel/packages/accounts-google/google.js#L5-9)

This is crucial for accounts-linkedin to be used with [accounts-merge](https://github.com/lirbank/meteor-accounts-merge) where original login* methods are called with a callback only (see [here](https://github.com/lirbank/meteor-accounts-merge/blob/master/accounts-merge-client.js#L18))

Please publish this at your earliest convenience.
Thanks a lot