Convert docs to reST

Add Varnish configuration chapter

Convert some more chapters to reST

Convert client chapter

Try to enable PHP syntax highlighting

Convert Nginx config chapter

prepare .rst doc
split varnish and nginx configuration into separate files

Fix some issues
Add Nginx config samples

Improve user context chapter

Update link to docs
Build docs on Travis

Fix references to glossary

Clean up
Merge pull request #73 from FriendsOfSymfony/rst-docs

Convert docs to reST