Add Google Custom Search box

Adds a Google Custom Search box to the header:

![Screeny Shot 10 Mar 2013 00 51 12](https://f.cloud.github.com/assets/227505/240475/a747b734-891c-11e2-9b1f-661a63c0516e.png)

Search is associated with mrdavidlaing@gmail.com's account; but other Google accounts can be given permissions to edit what is indexed etc.

Currently this custom search indexes:

1. http://cloudfoundry.github.com/*	
1. https://github.com/mrdavidlaing/cf-docs-contrib/wiki/*	
1. https://groups.google.com/a/cloudfoundry.org/forum/?fromgroups#!forum/bosh-dev	
1. https://groups.google.com/a/cloudfoundry.org/forum/?fromgroups#!forum/vcap-dev