Support for Blob object. BlobBuilder legacy support in an if block.

I followed the MDN example for changing BlobBuilder code to Blob, since webkit depricated BlobBuilder. Consequently, the html5-filesystem adapter broke in recent versions of Chrome.

The old code lives in an if block to support older browsers.

https://developer.mozilla.org/en-US/docs/DOM/Blob#Blob_constructor_example_usage

resolves issue: 

https://github.com/brianleroux/lawnchair/issues/147