Implement "sendtoname" in Qt UI

This patch implements an equivalent of the "sendtoname" functionality in the Qt UI.  You can now enter a name in the "Pay To" field, and have the coins sent to the address holding the name.  This only works if the name is not expired and 1-25 characters long.  The latter constraint is to prevent "typo squatting" on Namecoin addresses.

This may make live easier for "ordinary users" who don't like the cryptic addresses - and it can be used to tip websites or identities.