Clarify canonical import path, fix broken links.

The canonical import path for this package appears to be
"sourcegraph.com/sourcegraph/go-diff/diff", not
"github.com/sourcegraph/go-diff/diff". Add an import path comment to
enforce and document this. Also mention the import path in README, so
there's no need to guess what it actually is.
Fix broken views counter image.
Fix broken definition link.
Add Travis build status badge.
Update Travis Go to current version.

Author: dmitshur

.travis.yml

@@ -1,3 +1,3 @@
 language: go
 
-go: 1.3
+go: 1.4

README.md

@@ -1,13 +1,23 @@
-# go-diff [![status](https://sourcegraph.com/api/repos/github.com/sourcegraph/go-diff/.badges/status.png)](https://sourcegraph.com/github.com/sourcegraph/go-diff) [![views](https://sourcegraph.com/api/repos/github.com/sourcegraph/go-diff/.counters/views.png)](https://sourcegraph.com/github.com/sourcegraph/go-diff)
+# go-diff [![Build Status](https://travis-ci.org/sourcegraph/go-diff.svg?branch=master)](https://travis-ci.org/sourcegraph/go-diff) [![status](https://sourcegraph.com/api/repos/github.com/sourcegraph/go-diff/.badges/status.png)](https://sourcegraph.com/github.com/sourcegraph/go-diff) [![views](https://sourcegraph.com/api/repos/sourcegraph.com/sourcegraph/go-diff/.counters/views.png)](https://sourcegraph.com/github.com/sourcegraph/go-diff)
 
 Diff parser and printer for Go.
 
 **Unstable API:** go-diff is currently in development. If you depend on it, you should vendor it.
 
+Installing
+----------
+
+```bash
+go get -u sourcegraph.com/sourcegraph/go-diff/diff
+```
+
+Usage
+-----
+
 It doesn't actually compute a diff. It only reads in (and prints out,
 given a Go struct representation) unified diff output, such as the
 following. The corresponding data structure in Go is the
-[diff.FileDiff](https://sourcegraph.com/github.com/sourcegraph/go-diff/.GoPackage/github.com/sourcegraph/go-diff/diff/.def/FileDiff)
+[diff.FileDiff](https://sourcegraph.com/sourcegraph.com/sourcegraph/go-diff/.GoPackage/sourcegraph.com/sourcegraph/go-diff/diff/.def/FileDiff)
 struct.
 
 ```diff

diff/doc.go

@@ -1,2 +1,2 @@
 // Package diff provides a parser for unified diffs.
-package diff
+package diff // import "sourcegraph.com/sourcegraph/go-diff/diff"