pac4j-118: Improve documentation and core concepts

First commit
pac4j-118: Improve documentation and core concepts

header client is now abstract for real header client impl + README
pac4j-118: Improve documentation and core concepts

Updated after @miremond's code review
Merge pull request #119 from leleuj/pac4j-118

pac4j-118: Improve documentation and core concepts
Merge branch 'master' of https://github.com/pac4j/pac4j
Some fixes to Strava OAuth2 integration according to the code review by Jérôme.