adding structured data to base theme

This adds metadata to the head element based on schema.org which Google, Bing and Yahoo will like. The cover page is identified as a [schema.org/Book](http://schema.org/Book) and the rest of the pages are identified as [schema.org/WebPage](http://schema.org/WebPage). 
* I chose to put the microdata in the head element and link it with `itemref`to be non-intrusive should the templates change. 
* These changes will cascade through to all child themes that use 'Luther' as a parent if those child themes don't override any of the template files. 
* I created shortcuts to the functions should other themes/theme designers also want to make use of the functionality. 

![screen shot 2014-06-19 at 7 51 31 am](https://cloud.githubusercontent.com/assets/2048170/3329129/dfce1550-f7c3-11e3-807f-f14bba981633.png)

This is the output of the above cover page using Google's 'Structured Data Testing Tool': 

![screen shot 2014-06-19 at 7 52 20 am](https://cloud.githubusercontent.com/assets/2048170/3329263/263f04ee-f7c5-11e3-8799-1a65f077eae8.png)