handle image extensions in doctrine loader

This is a first step towards #78 - we try to lookup an image also without file extension. this is more of a content negotiation thing than of proper  ids, so @lsmith77 and i feel it belongs here rather than for example https://github.com/symfony-cmf/MediaBundle/blob/master/Doctrine/Phpcr/MediaManager.php#L121

with this change it will become possible to at least build your image urls with a hardcoded format, e.g. use `format: jpg` in the filter definition, or put it into the output like ``{{ item.image.id | imagine_filter('header_desktop') }}.jpg`` 

/cc @rmsint