add coveralls integration

secure is travis's way of hiding security information, and the token can be generated from the travis command line utility, via `travis encrypt`.  The token itself comes from coveralls.io, along with the suggestions for before_install and the call at the end of script.
fix go test call

Travis said the build failed, probably because of the wildcard.  This approach is exactly what coveralls.io suggests to do.
update token

previously had no $ in front of the environment var.
correctly use secure environment variable

... I think
move the repo token export to the before_install stanza

third time is the charm?
Update .travis.yml
this is getting old...
Add coverage flair to readme.md
Merge pull request #21 from gonum/add-travis-ci

add coveralls integration - this adds a bit of flair to the readme.md.