Improve field arguments to make files always accessible by ID  

BUGREPORT
$this->field->object_type() : expected return -> (custom) Posttype : actual return -> false (always)
$this->field->object_id() :  expected return -> (custom) Post-ID : actual return -> false (always)
both methods are not implemented,  __call fallback accesses wrong properties
Postmeta READ always fails (no ID/type), fallback using GUID fails on edited/moved attachments
http://pods.io/2013/07/17/dont-use-the-guid-field-ever-ever-ever/  
RESULT: missing images on e.g. edit.php ( for GUID != filename ) 
FIX: replace function calls with simple properties
Merge pull request #70 from trelvox/trunk

Improve field arguments to make files always accessible by ID