Refactor of image model

*Refactored __init__ to account for correct properties
*Removed datetime formating from __repr__
*Removed true and false formatting from _json_to_obj
*Refactored _dict_to_obj to account for not only correct properties, but also additional properties
*Refactored _obj_to_json to account for correct properties
*Fixed formating to be consistent
*Added summary documentation for each class
*Fixed checksum assignment
*Removed direct_url
*Added TODO tag to note the additional of direct_url and locations at a later date
*Added explanation of for loop for filtering  additional properties

Change-Id: If53ebb21b336e4975239f2514ff8322972a847fb
Merge "Refactor of image model"