Support for Nintendo handhelds DSi and 3DS

Added support for Nintendo handhelds DSi and 3DS / their browsers into user-agent detection; so that they display the mobile theme
Would this be wiser to do as `stripos` for case insensitive matching?

Also, perhaps just match nintendo, to catch their declared useragent:

`Mozilla/5.0 (Nintendo 3DS; U; ; en) Version/1.7498.US`

as per http://www.nintendo.com/3ds/internetbrowser/specs/