Adds CallSource nodes to the control flow graph, which have the same role for backwards analysis as the CallReturn nodes have for forward analysis.

adds nice explanatory images to the documentation of `graph.rs`.

* added links to online documentation, added black hat slides

* add spaces for codacy

* This commit improves the cwe_checker_to_ida tool. First, it fixes
issue #24. Second, it introduces some unit tests for
cwe_checker_to_ida. Third, cwe_checker_to_ida parses newer cwe checks
like cwe415 or cwe787. Forth, updated description of
cwe_checker_to_ida in README.md.