time_from and time_till parameters to Item::history are now optional.
HostGroup now has hosts accessor.
Basic support for the User class.
Work in progress for the UserGroup class.

Most tests pass in 1.8.5, curious if everything passes in 1.8.later.
Updated User class for UserGroup accessor. Fixed doc.
Fixed UserGroup POD.  It's not a copypasta of HostGroup's anymore.
Merge branch 'master' of https://github.com/fgabolde/Zabbix-API