Add doxygen command \agentxcpp.

The command can be used to reference items in the agentxcpp namespace,
like so:
\agentxcpp{MasterProxy}
This will produce the Text "MasterProxy" which links to the MasterProxy
class.
TableEntry::variables() returns QMap, not QVector.
Bugfix: Check iterator after map::upper_bound().

And after map::lower_bound(), too.
Add NULL-pointer checks to Table::addEntry().
Fix OID;s of table objects.
Add the TableEntry::subid member.

The member represents the subid of the entry as given in the MIB.