Add module to support multiple connections
Add a method to close all connections
Don't instantiate multiple connections for a host
Add a property to get the number of instantiated connections
Fix syntax error
Store hostName and port to the connection object itself
Use given host name prior to the host specified by an environment variable DROONGA_ENGINE_HOST.

To support multiple connections, the global environment variable have to be ignroed.
Add tests for Connections
Add new utility method to create stubbed backend connections
Support multiple connections for multiple droonga-engine hosts