Add Content-Security-Policy header

This would mitigate any possible XSS attack since the browser won't execute any Javascript.
Add test
Fix typo
Merge pull request #325 from LukasReschke/master

Add Content-Security-Policy
Adding support for calendar subscriptions.
Plugins, nodes, backends.
Working in iCal. Needs unittests.
Added ACL
Test for the plugin.
More unittests.
Small fixes.
More unittests.
Fixed security problem for windows machines.
Merge branch '1.6' into 1.7
Updated changelog
Merge branch '1.7' into 1.8

Conflicts:
	ChangeLog
	lib/Sabre/DAV/Browser/Plugin.php
Few smallish fixes.
Fixed a few problems in the unittests. CalDAV is back to 100% coverage.
Set release date
Merge branch '1.6' into 1.7

Conflicts:
	ChangeLog