Changed DNSClient ctor to require response param be
an empty pointer and clarified related commentary.
Added debug log statement to log transaction start.
Other minor cosmetics.

Now that NameAddTransaction and NameRemoveTransction classes
have been implemented, D2UpdateMgr has been updated to use
them.  It now creates instances of NameAddTransaction and
NameRemoveTransaction based on the change type specified in
received NameChangeRequests as designed.

Good deal of commentary clean up, rolled back IOService reference
changes to DNSClient, and most significantly added unit tests
for NameChangeTransaction::sendUpdate.  These tests involve an
actual message exchange.

Some classes were using references to isc::asiolink::IOService, others
where using d2::IOServicePtr.  The latter is now used throughout for
consistency as well as support for future, possible, multi-threaded
implementation.

D2UpdateMgr members were rescoped to be more appropriate
and unit tests adjusted accordingly. Other minor changes.

Added initial implemenation of D2UpdateMgr class to src/bin/d2.