Move release building automation from top-level Makefile to main
PRISM Makefile and delete the top-level one: after switching to git,
the required top-level directories (prism-examples, cudd, etc.) are
always present and there is no need for the separation.
Source releases will now be direct copies of the whole git repo. The Makefile
targets for building binary releases have been simplified and updates. They
are now triggered by "make release" or "make release VERSION=4.5".
There is also a new makefile target "doc" that currently pulls down a copy of
the online manual and stores it in the "manual" directory.