Smarter regex and more tests

• #69 If src image doesn't exist, return a 404 exception.
• #58 Adding unit tests for generate()
• #13 Restrict the route listener regex to just handle images found in a src_dir
• Adding other unit tests and making many private methods public
• Dropping Travis CI support for PHP 5.3 and adding PHP 5.6
• Croppa will regenerate the cropped version of files if generate() is called for a path that already exists
• generate() no longer outputs the image when it finishes. Instead, it returns a Bkwld\Croppa\Image object with a show() method that will echo the binary image data