feat(travis): add coveralls support

.travis.yml

@@ -5,4 +5,9 @@ go:
   - release
   - tip
 before_install:
+  - go get github.com/axw/gocov/gocov
+  - go get github.com/mattn/goveralls
+  - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
   - curl -s https://raw.githubusercontent.com/lovell/sharp/master/preinstall.sh | sudo bash -
+script:
+    - $HOME/gopath/bin/goveralls -service=travis-ci