Fixed feature detection in Webkit and global decl.

Fixed #22 and #24 and changed Blob declaration to method of window
object (=global), which makes it work in Meteor environment.

Tested on:

OS X:
 - Opera 11.62
 - Opera 21.0
 - Safari 7.0.3
 - Chrome 35.0
 - Firefox 3.6
 - Firefox 10.0.4
 - Firefox 29.0.1

iOS 7:
 - Safari
 - Opera mini 7.0.5
 - Opera Coast 3.0.1
 - Chrome 30.04

Windows 7:
 - Safari 5.1.7
 - IE 9