Update gravatar-image.js to not use prototype functions

This is needed for those with prototypes turned off.  Also, `alt` was not observing `email` so the computed was not updated if `email` changed.

http://emberjs.com/guides/configuring-ember/disabling-prototype-extensions/
Merge pull request #12 from jasonmit/patch-1

[Enhancement] Update gravatar-image.js to not use prototype functions