Add deep path support on orderBy

[digitake]

Dear Pyrebase team,

I happen to use pyrebase module for my project. However, one of the feature i need is to order object by deep key path. This is already support by new version of firebase but doesn't support by the module yet.

So i submit the code to support this.

db.child("somepath").order_by_child('a/b/c')

the line above will not throw an error with my patch.

[faiza-nawaz-confiz]

It's not the part of the current module. Is it merged?