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
Minor fixes to match coding style
Moved protected method to bottom
Bumped to 2.5
Added version check so example won't fail on PHP < 5.4
Updated .gitignore
Fixes #31
Better composer usage