Accept a comma separated string for servers

In the client, accept as and argument either a comma separated string of
servers, or an array.

When the environment variable is set to a comma separated string, this already works fine, but if the programmer passes in the same string (e.g. with `ENV["MEMCACHE_SERVERS"]` explicitly), it's not parsed. This changes/fixes that by parsing immediately in the constructor instead of waiting until `ring` is called.