Fix tags config

Fix "PHP Notice:  Undefined index: tag_listener in friendsofsymfony/http-cache-bundle/DependencyInjection/FOSHttpCacheExtension.php
on line 73" when ExpressionLanguage is not available.
Merge pull request #112 from FriendsOfSymfony/fix-tag-config

Fix tags config
Fix invalidation config
Merge pull request #113 from FriendsOfSymfony/fix-config-invalidation

Fix invalidation config
Make tags header configurable (fix #73)
adjust test to now also testing xml
Merge pull request #109 from FriendsOfSymfony/tags-header-config

Make tags header configurable
Refactor docs

Fix #94.

Add bundle activation to installation

Improve @Tag section in Annotations

Include match partial in invalidation chapter

Update user context docs

Fix typos

Add chapter intros

Add chapter headers

Add Debug config chapter

Add flash message chapter (todo)

Update proxy client chapter

Remove default config chapter

Add Configuration chapter header

Re-organise configuration chapters

Match is now linked to instead of included, and included in the TOC too.
various tweaks and cleanups
Finalise docs

* fix links to FOSHttpCache docs
* a little clean-up