Made cqerl:new_client more flexible on its first argument

Now it can handle single IP address (list or tuple), without being in a {IP, Port} tuple. It will default to the default port for cassandra's native protocol
Added a named connections

Now, by providing a `name` option in either `new_client`, `prepare_client` or in environment variables, you'll be able to simply do `cqerl:new_client(Name)` to get a client
Fixed a bug in cqerl_client