Add support for 2.8.5 and multi-arity commands

This is the start of big changes for er.

er now directly implements all commands as of Redis 2.8.5 (some
of the new commands aren't hooked up to erlang-specific type
decoders yet.  Fix as necessary).

er now also generates multi-arity commands for Redis commands
that support var args.  Example: er now generates multi-functions
like: er:mget/3, er:mget/4, er:mget/5, ..., er:mget/16. (16 by
default, but you can increase it arbitrarily high *or* you can
just add a list of the other arguments you want as well.)

er maintains a list of Redis functions that aren't var arg
capable (like GET versus MGET), so the fixed-arg functions
don't get multi-arg counterparts.

Also, er_server.lfe is gone because er_pool is the proper way
of creating named and long-lived connections to your Redis server.

Coming soon: erlang maps instead of proplists for some return
values, moving to poolboy for the connection pool, support
for sentinel, cluster, and live failover mechanisms.