Merge pull request #80 from hpatoio/master

Moved Varnish specific information to Varnish page
Require at least one context provider (fix #77)
adjust doc to hash context change.
Merge pull request #82 from FriendsOfSymfony/hash-generator-providers

Require at least one context provider
cleanup doc to not have sidebar along code
Make context provider setter private
Switch to SVG for Travis tag
Restricted the Travis builds to some branches only

We don't want Travis to build feature branches created in the main repos, as these feature branches will always have a PR which is built as well.
Travis will now run builds only for the master branch and for branches named "x.y" where x and y are numbers (to let us create maintenance branches for releases if needed), and for pull requests sent to these branches.
test user context error situations
Merge pull request #83 from FriendsOfSymfony/test-context-errors

test user context error situations
fix typo and doc highlighting
do not use x-original-url for backing up the url. fix #84
Merge pull request #86 from FriendsOfSymfony/lookup-origin-url

do not use x-original-url for backing up the url.