[MONDRIAN-1448] Expose monitoring metrics via JMX MBeans

Adds a MonitorMXBean interface which defines what will be exposed,
registers that bean in MondrianServerImpl, and adds getters for
the attributes in the various *Info classes (e.g. ServerInfo)
Merge pull request #264 from mkambol/mondrian-1448

[MONDRIAN-1448] Expose monitoring metrics via JMX MBeans