Implement (*Conn).Unexport
Export InvalidTypeError; don't panic that often
Handle calls on closed connections better

Previously, calls on a closed connections would panic, mostly because of
sends on the closed output channel. Now, a special error is returned if
a connection is already closed. This requires a RWMutex to protect the
channel from being closed while a method tries to send to it. It also
changes some methods to returns errors.
Emit: Check that parameters are valid
prop: Send more context with property change notifications

This changes the type of the channel passed to prop.New to a new type,
Change, which carries more information about what was changed so a
single channel can be used for multiple properties.
prop: Add Check member to Prop

Also, change the map argument of New to map[string]map[string]*Prop.
Send better error message for missing object
Report correct signal name
prop: Change callback / validation mechanism

Since checking for the validity of incoming Set calls, if needed, must
be done synchronously anyway, there's no reason not to just use a
function that checks the argument for validity and / or serves as a
callback to notify of changes.
introspect: Ignore unexported methods for Methods
Fix 2 typos