Update staff client's about page to include newest core committers

Update staff client's about page to include newest core committers

The newest core committers were elected during the Evergreen Hack-A-Way on
October 10, 2012:

Jeff Godin, Traverse Area District Library
Benjamin Shum, Bibliomation, Inc.
Jason Stephenson, Merrimack Valley Library Consortium

Also update language of page to reflect that these contributors are now core
"committers" rather than "developers".  And move past core committers into
their own section.

Signed-off-by: Ben Shum <bshum@biblio.org>