Add attribute for maximum object size

Default to 1M in the attribute, which is the default for memcached.
This feature requires version 1.4.2, which is satifisfied in Ubuntu
10.04-current and RHEL 5 to current.  It's been out for a while so this
should really be on any mildly modern OS.
Merge pull request #12 from tas50/COOK-2790

[COOK-2790] Add attribute for maximum object size