Use Git Submodule to include the pickadate.js

Hi, this is a dummy PR because you don't open the issue function.

But I think yours fork is better than the [origin one][1].
While a thing can be better, use Git sub-module to include the official package. Just like [momentjs][2] does.

[1]: https://github.com/mervas/meteor-pickadate
[2]: https://github.com/crapthings/meteor-momentjs/blob/master/.gitmodules

If you don't like that, please ignore me.