Consolidate protocol messages in one namespace

Rather than defining the various messages as nested classes in SignalProxy,
RemoteConnection and in the future even more locations, have them defined
within a single namespace (and file, protocol.h).
This should make it much easier to see at a glance what kind of messages
Quassel sends, and thus help with protocol implementations.
Allow compilation against Qt < 4.7 again

Looks like I recently introduced the use of some QDateTime stuff that appeared
in Qt 4.7. Let's use the old way of calculating the lag then if built against older
Qt...

Oh, also this fixes that we would show double the lag.
Yearly bump

It's 2013, baby!