Config gopath overrides

adds two new shell variables which are respected

KUBE_EXTRA_GOPATH and KUBE_NO_GODEPS

The first will be added to the final GOPATH.  The later will cause the Godeps directory to not be added to the final GOPATH.  Used in conjunction these can be used to replace the Godeps with something outside of the tree.
Can you include "from KUBE_EXTRA_GOPATH" in this comment?

We've been using the standard of starting comments with capital letters and ending in a period, it would be great if you kept it consistent.