Implement reconnection strategy class

Extend base Connection class to support a list of nodes and an
optional Strategy parameter to choose next item from this list.

Add built-in reconnect strategy class based on Round-Robin alg.
@params:
- addrs, list of maps {'host':(HOSTNAME|IP_ADDR), 'port':PORT}.

Return next connection or an error if all URIs are unavailable.

Closes #106