add Google Nexus 7 (2013)

Google Nexus 7 Tablet added. Old Nexus 7 renamed as Nexus 7 (2012).

Sources: http://bjango.com/articles/min-device-pixel-ratio/
http://www.google.com/nexus/7/
Merge pull request #11 from emrehan/patch-1

add Google Nexus 7 (2013)