Export the AV changed signal
Atomspace: Change signature of the merge signal.

Also, the OAC was asking for and getting the wrong signal ...
atomspace: fix bad indentation
oops, typo in previous commit
bugfix: decaySTI was not issueing a signal.
UTests: change to the new signal signature
atomspace: define a type signal.
Merge pull request #357 from linas/master

Fix AVChanged signal; tweak TVChanged signal