Travis: Run gofmt -s, go vet, go test -race, add Go 1.6.

Also make tip a fast-finish allowed failure. That way, if CI fails on
tip due to a temporary issue with tip, it will not break build status.
However, it's still possible to see tip build status by looking at CI
details page.

Do not run go vet with Go 1.4 or older since it's not included in the
standard library, and it's no longer available in external standard
library.

Add godoc badge to README.md.

.travis.yml

@@ -1,18 +1,30 @@
-# Travis CI (http://travis-ci.org/) is a continuous integration service for
-# open source projects. This file configures it to run unit tests for
-# blackfriday.
-
+sudo: false
 language: go
-
 go:
-    - 1.2
-    - 1.3
-    - 1.4
-    - 1.5
-
+  - 1.5.4
+  - 1.6.2
+  - tip
+matrix:
+  include:
+    - go: 1.2.2
+      script:
+        - go get -t -v ./...
+        - go test -v -race ./...
+    - go: 1.3.3
+      script:
+        - go get -t -v ./...
+        - go test -v -race ./...
+    - go: 1.4.3
+      script:
+        - go get -t -v ./...
+        - go test -v -race ./...
+  allow_failures:
+    - go: tip
+  fast_finish: true
 install:
-    - go get -d -t -v ./...
-    - go build -v ./...
-
+  - # Do nothing. This is needed to prevent default install action "go get -t -v ./..." from happening here (we want it to happen inside script step).
 script:
-    - go test -v ./...
+  - go get -t -v ./...
+  - diff -u <(echo -n) <(gofmt -d -s .)
+  - go tool vet .
+  - go test -v -race ./...

README.md

@@ -1,4 +1,4 @@
-Blackfriday [![Build Status](https://travis-ci.org/russross/blackfriday.svg?branch=master)](https://travis-ci.org/russross/blackfriday)
+Blackfriday [![Build Status](https://travis-ci.org/russross/blackfriday.svg?branch=master)](https://travis-ci.org/russross/blackfriday) [![GoDoc](https://godoc.org/github.com/russross/blackfriday?status.svg)](https://godoc.org/github.com/russross/blackfriday)
 ===========
 
 Blackfriday is a [Markdown][1] processor implemented in [Go][2]. It