Add gc (Go compiler) as Go linter

Added a new linter class to invoke gc to compile Go code and extract any
resulting compile error messages. If the gofmt package is also
installed, the code will be formatted before being compiled. This
probably means that the linter won't catch syntax issues. A future fix
could be to first invoke gofmt and then gc if it doesn't fail. This
would require uninstalling the gofmt package.
Check for user-provided path to `go` tool.
Clean up.
Merge pull request #23 from moshee/master

Add Go linter