Avoid accessing memcached_server_st struct member directly and use accessor functions instead in versions of libmemcache that support them. Works around a bug in libmemcache where the externally accessible server struct definition is out of sync with the internal one.
Additional fixes for Issue #112.
It was not caused by a bug in libmemcached, but was due to an interface change introduced in libmemcached-0.39. The memcached_server_function signature was changed to take a memcached_server_instance_st as its second argument instead of a memcached_server_st*. memcached_server_instance_st is aliased to a void* in the C interface headers and maps to a C++ struct on the implementation side and replaces the old memcached_server_st struct in some interfaces (but not all).
Merge pull request #113 from kelihlodversson/master

Proposed fix for issue #112