Add cordova detection

should fix #53

Since 1.7 `cordova` is exposed instead of phonegap. 2nd secure test is that protocol is `file:` but should not be used alone because a user can open a file with its own browser too.

cordova could be use on a lot of device and in many case preserve original UA so I kept the `_addClass` outside the if else logic to be able to detect both ios & cordova for example.