Added the ability to manipulate base64 image strings

Allows the user to input a base64 image string instead of image.  User
could modify the image using <canvas>, upload the client side
manipulated base64 string and go from there.  Requires PHP 5.4 for
getimagesizefromstring

How to use :

$img->load_base64($base64string)->resize(32,32)->output();
Changed exception wording

Now explains that PHP 5.4 is required
Merge pull request #29 from mikedidomizio/master

Added the ability to manipulate base64 image strings