Always expose D3 onto the environment global

With regards to #1689 adding AMD compatibility, it seemed odd to me that
D3 opt'd out of exposing itself globally for third-party plugins.

Within a discussion about this very problem, found here:
https://github.com/jakevdp/mpld3/issues/33#issuecomment-32101013

the argument is made that exposing the global defeats the point of
module systems.  I agree that module systems help avoid global
pollution, but that is not something I'd consider the point of module
systems, especially since they are registered in a global namespace
themselves.

This patch will fix third-party modules that are shimmed.