CSP header format dependent on user agent

Using the correct format is essential for the browser to accept the CSP.
This is particularly important for Firefox < 23 as it used a different set of
directives.
The option 'enforcePolicy' has been added to determine whether unrecognised user agents
should receive all 3 main CSP header formats (enforcePolicy=true) or none.
The main user agents have been added at this stage but the list should be refined to
include other browsers (especially mobile browsers) that support CSP.
Merge pull request #1 from edgecampus/browser-specific-csp

CSP header format dependent on user agent