Break libgearman up

RIght now libgearman contains the client, worker, server, and gearmand code in one library. It may make sense to break this into:

libgearman - core, client, worker
libgearman_server - generic server components built on libgearman core
libgearmand - embedded gearman job server that uses libgearman_server

This als would allow optional compiles of libgearman only, with less dependencies (no libevent).