Upstream at go.tools are breaking things, so fix

tl;dr https://github.com/dominikh/go-importer/blob/master/importer.go#L112 should be types.Config not types.Context see[0]

I'm not sure if you're getting to it, but when I tried to 'get' a package that uses this one, I get an error. I see that you're already adjusting to the changes, but I'll go ahead and mention one that I ran into. They changed types.Context to types.Config. 

[0] http://code.google.com/p/go/source/detail?repo=tools&r=5a9c2a38449e3b024dd12d98e2f33e686d9f95a0&spec=svn.tools.e5e9ae03f5d14c1eb1b6d4982e3ad33c9470a299#publish