Python3 support

FYI in case any one is interested. Works on Go 1.1-ish as it's not been released yet, but "go version devel +b27b1ff18f39 Wed Apr 10 07:15:49 2013 +0200 darwin/amd64". Also tested on darwin/386 and linux/amd64. Other platforms untested.

And if you're not interested in Python 3, there's also a memory leak fix in https://github.com/quarnster/gopy/commit/8649515062388d59ae4be3b9f3f056a9170eb396 that should be applied to all versions.