solve all Eclipse warnings

Eclipse warns if a variable is defined but not used.
resolve yml validator warning in csp.yml
nelmio_security.csp_listener can have global scope

The listener does not depend on any other service or input that changes
with each request.
Make ContentSecurityPolicyListener an EventSubscriber
make configuration tree for directives reusable

In a next step I want to add two config subtrees to define
directives to report and to enforce, see #31
introduce DirectiveSet class

This commit changes the constructor of ContentSecurityPolicyListener
but I don't consider this to be a public API. The public API is just
the configuration.
add config def for report|enforce subtrees

but use the full directive names
e.g. img-src instead only img, report-uri instead report_uri
handle new configuration with report, enforce sub trees
update README for new csp configuration
fix coding style issues

I ran phpcs with standard PSR2 and fixed all issues introduced by me or in
files touched by me. There are however still many issues in other files.
I also fixed the issues outlined in the line comments in my first PR.
Merge pull request #33 from comsolit/master

fix #31 please make it possible to report and inforce policies