* new module swift.obj.diskfile
I parameterized two constants from obj.server into the DiskFile's __init__
* DATADIR -> obj_dir
* DISALLOWED_HEADERS -> disallowed_metadata_keys
I'm not sure if this is the right long term abstraction but for now it avoids
circular imports.
Change-Id: I3962202c07c4b2fbfc26f9776c8a5c96292ae199