Better detection for Tablet UAs.

This change allows you to correctly detect the following browsers as tablets:

Firefox OS tablet: 
`Mozilla/5.0 (Tablet; rv:29.0) Gecko/29.0 Firefox/29.0`

Blackberry Playbook: 
`Mozilla/5.0 (PlayBook; U; RIM Tablet OS 2.0.1; en-US) AppleWebKit/535.8+ (KHTML, like Gecko) Version/7.2.0.1 Safari/535.8+`

Opera Mobile (running as tablet): 
`Opera/9.80 (Android 2.2.2; Linux; Opera Tablet/ADR-1111101157; U; en) Presto/2.9.201 Version/11.50`