Test Handle use count: first rough draft

Introduce a new unit test, to check for the strangeness
that Nil is seeing.
UseCountUTest: work on implemenmtation
Atomspace: additional documentation
UseCountUTest: continued development
final form of the use-count test case
UseCountUTest: polish it up a bit
remove unused file
Merge pull request #1110 from linas/tom

Test multi-threaded STL use count