Some Makefile clean up

1. Use the @ modifier on commands so that they don't get printed

2. Use target-specific variable for the name of the coverage file
   so that it can be changed without changing in multiple places.

3. Use target-specific exported variables to put GOOS and GOARCH
   into the environment for go build.
Merge pull request #4 from jgrahamc/master

Some Makefile clean up