Report when both X/CSP and X/CSP-Report-Only are present. See #279.

I have made the render_reponse shortcut in test_csp2.py to take list of types
instead only adding one header per response.
Report when unknown directives are detected. See #279.

This is a messy commit. I will reoganize the code and refactor later.
For now, I am only reading policies from CSP. I will work on reading
other X/CSP headers in future commit.
When deprecated directives are encountered, report them to owner. See #279.
Tell how many inherit from default-src. See #279.
Report issues related to source list. See #279.
Remove legacy code in CSPPlugin
Added some verbose descriptions in CSPPlugin. See #279.