Update install instructions
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