omcache_pylibmc: handle value deserialization in get_multi
compatibility wrappers: add ASL-2.0 copyright notice

Although they can now be distributed in places where Public Domain doesn't
exist it doesn't mean that these wrappers are supported or that they should
be used.
server info: make it versioned and allow null command in omcache_stat

Server info struct now has a version number to make it possible to add
members to it in a backwards compatible way.  It also gained a
'server_index' member which is the OMcahce internal server id and can be
passed on to omcache_stat and similar functions.

Passing NULL to omcache_stat is now equivalent to passing it an empty string
which, on most memcached server implementations, returns the default set of
statistics.