added a module to provide a factory to get an appropriate connector.

* introduced a module jsch-agent-proxy-connector-factory,
* introduced com.jcraft.agentproxy.ConnectorFatory,
* introduced the static method ConnectorFatory.getDefault() to get a connector.

https://github.com/ymnk/jsch-agent-proxy/pull/10
fixed a bug in checking the availability of usocket factories.
Keep refrain from using foreach syntax.
The examples should be under src/main/java/com/jcraft/jsch/agentproxy/examples/.
fixed a typo in the definition of ConnectoFoctory#setPreferredConnectors()
Use ConnectorFactory.
Merge branch 'issue10/connector_factory'