[server][HAPI JSON] Support host information in event

This change is for Hatohol 14.12. Please keep it open until 14.09 is released.

TODO: This implementation doesn't work on multi-master architecture
because new host ID is emitted on each Hatohol server.
In this situation, other similar code in this project uses 'const HostInfo &hostInfo = *it' to avoid hostInfo from begin updated wrongly.