add a basic printer into graphviz (http://graphviz.org/) for proofs

- proofs are output into file `proof.dot` if `(get-proof-graph)` is
  in the input
- use `dot -Txlib proof.dot` to see the proof
- use `dot -Tsvg proof.dot` to get a svg file

src/ast/CMakeLists.txt

@@ -10,6 +10,7 @@ z3_add_component(ast
     ast_printer.cpp
     ast_smt2_pp.cpp
     ast_smt_pp.cpp
+    ast_pp_dot.cpp
     ast_translation.cpp
     ast_util.cpp
     bv_decl_plugin.cpp