Run the Python tests on our continuous build.
Avoid the dependency on the makefile from Go.

This avoids the extra step of "make golibs", and also leaves Go in control of
compilation flags.
Add a .md extension to go/README.
Update the content of go/README.md to Markdown.
Convert go/README to Markdown format.
Remove the nearly useless go/ENV file.
Reformat merkle_tree_go.cc.