Travis: Perform gofmt, go vet checks; use race detector during tests.

This change augments the Travis CI build to perform:

- Check that all files follow gofmt style, including -s (simplify) option.
- Check that go vet does not report any problems.
- Use race detector when running tests, to ensure there are no data races found.

.travis.yml

@@ -1,8 +1,14 @@
 language: go
 sudo: false
-
 go:
   - 1.3
   - 1.4
   - 1.5
   - tip
+install:
+  - go get golang.org/x/tools/cmd/vet
+script:
+  - go get -t -v ./...
+  - diff -u <(echo -n) <(gofmt -d -s .)
+  - go tool vet .
+  - go test -v -race ./...