[Dev] README: contentmodel moved; update personal parcels

[Phillip J. Eby]

I've just checked in changes to move osaf.contentmodel -> osaf.pim.  Please 
note that many modules were renamed in the process, e.g. ItemCollections -> 
item_collections, and Collections -> collections.

(Please do not create new modules in the osaf.pim tree that do not conform 
to the new naming convention; module names should be all lowercase and 
plural in form, to help new developers readily distinguish module names 
from class or method names.)

 From now on, when accessing classes in osaf.pim, you should just use "from 
osaf import pim" and then use pim.ClassName, wherever practical.  For 
example, "from osaf import pim; pim.Contact".

The only times you should need to directly import from a module inside of 
osaf.pim is if:

1. You are writing code *inside* osaf.pim, or

2. You are working with pim subpackages that haven't been "flattened" yet, 
like calendar, mail, and photos.  (This list will shrink over time.)

Be sure to run "make purge" or tools/purge.py on your next 
update.  Finally, note that if you have personal parcels referring to 
"contentmodel" in their XML namespaces, you must change them to say "pim" 
instead now.