shill: Implement a basic IPConfig class. Different IP configuration types will inherit from that class -- e.g., DHCP clients, PPP, etc. BUG=chromium-os:15953 TEST=unit test Change-Id: Id6ba4559afdf0b10dac43fed55ab51d272aeb4df Reviewed-on: http://gerrit.chromium.org/gerrit/1852 Tested-by: Darin Petkov <petkov@chromium.org> Reviewed-by: Paul Stewart <pstew@chromium.org>