Add build contrains to exampls to avoid error with App Engine.

I used svgo with App Engine to [draw braille][1].
The svgo is great! Many thanks!!

But, I had a problem which cause of App Engine trying import svgo's examples too.
To avoid this I had to remove all examples from svgo for App Engine.

Now, from [App Engine 1.7.4][2], It is can choose some files
not to be used by App engine with build constraints.

This patch add following build constraints line;

    // +build !appengine

to every examples.

[1]:http://braille-printer.appspot.com/
[2]:http://blog.golang.org/2013/01/the-app-engine-sdk-and-workspaces-gopath.html