Let the Makefile figure out which version of Go to download

The `Makefile` should figure out which file to download given only the Go version number.

I've modified the `Makefile` to derive the right name of the Go release file based on the information from `uname`. I seems to work on the two computers I have access to (one Linux and one OS X).

Current limitations:
* Only tested with Linux and OS X
* Special handling of OS X which might be too fragile to be future proof.