* Node::isAtomic() now looks at an "atomic" attribute of arguments

* Node::isAtomic() now looks at an "atomic" attribute of arguments
  instead of assuming it's atomic based on kind.  Atomicity is
  determined at node building time.  Fixes bug #81.  If this is
  determined to make node building too slow, we can allocate another
  attribute "AtomicHasBeenComputed" to lazily compute atomicity.

* TheoryImpl<> has gone away.  Theory implementations now derive from
  Theory directly and share a single RegisteredAttr attribute for term
  registration (which shouldn't overlap: every term is "owned" by
  exactly one Theory).  Fixes bug #79.

* Additional atomicity tests in ExprBlack unit test.

* More appropriate whitebox testing for attribute ID assignment
  (AttributeWhite unit test).

* Better (and more correct) assertion checking in NodeBuilderBlack.

* run-regression script now checks exit status against what's provided
  in "% EXIT: " gesture in .cvc input files, and stderr against
  "% EXPECT-ERROR: ".  These can be used to support intended failures.
  Fixes bug #84.  Also add "% EXIT: " gestures to all .cvc regressions
  in repository.

* Solved some "control reaches end of non-void function" warnings in
  src/parser/bounded_token_buffer.cpp by replacing
  "AlwaysAssert(false)" with "Unreachable()" (which is known
  statically to never return normally).

* Regression tests now use the cvc4 binary under
  builds/$(CURRENT_BUILD)/src/main instead of the one in bin/ which
  may not be properly installed yet at that point of the build.
  (Partially fixes bug #46.)

* -fvisibility=hidden is now included by configure.ac instead of each
   Makefile.am, which will make it easier to support platforms
   (e.g. cygwin) that do things a different way.

* TheoryUF code formatting.  (re: my code review bug #64)

* CDMap<> is leaking memory again, pending a fix for bug #85 in the
  context subsystem.  (To avoid serious errors, can't free context
  objects.)

* add ContextWhite unit test for bug #85 (though it's currently
  "defanged," awaiting the bugfix)

* Minor documentation, other cleanup.