accept header content negotiation

* look at wildcards */* and main/*
* return a 406 when nothing matches the accept header
Merge pull request #523 from yesodweb/content-negotiation2

accept header content negotiation
Persistent 1.2
Include persistent 1.2 in sources.txt
Ignore vendor
Merge branch 'yesod1.2' of https://github.com/yesodweb/yesod into yesod1.2
Merge branch 'json-auth2' of https://github.com/yesodweb/yesod into yesod1.2
Expose YesodExample