Add a Go language example.

This follows the other examples so that it can be used as a tutorial,
such as the ones at:
  https://developers.google.com/protocol-buffers/docs/tutorials

Even though Go generally does not use Makefiles, I added targets for the
Go examples to be consistent with the other languages.

Edit:

Fix Travis run. Change to use $HOME instead of ~. Add protoc to path.
GOPATH entry cannot start with shell metacharacter '~': "~/gocode"

Edit(2):

Fix Go code style to address comments.
diff --git a/.travis.yml b/.travis.yml
index 7be0d18..7780174 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,6 +15,7 @@
   - CONFIG=cpp
   - CONFIG=cpp_distcheck
   - CONFIG=csharp
+  - CONFIG=golang
   - CONFIG=java_jdk6
   - CONFIG=java_jdk7
   - CONFIG=java_oracle7
@@ -48,6 +49,10 @@
     # which doesn't work on OS X.
     - os: osx
       env: CONFIG=csharp
+    # Requires installing golang, currently travis.sh is doing that with apt-get
+    # which doesn't work on OS X.
+    - os: osx
+      env: CONFIG=golang
   # Add into the matrix OS X tests of Objective C (needs Xcode, so it won't
   # work on other platforms). These are split so it doesn't take as long to run.
   include: