Extend recipe for go to setup typical dependencies

* GOPATH per http://golang.org/doc/code.html#GOPATH
* Mercurial (hg) for go get
* Bazaar (bzr) for go get
* Modify PYTHONPATH for hg & bzr to work correctly on OSX
Add $GOPATH/bin to $PATH

For convenience, per http://golang.org/doc/code.html#GOPATH
Add cross-compilers and runtime support

Most of the go projects on CF use this