= io: register for OP_CONNECT only after the connection attempt was made, fixes #437

This is necessary because of at least two bugs in the JDK:
 * a channel is always connectable before the connection is attempted,
   and selecting for OP_CONNECT will instantly report connectable
 * even worse: after OP_CONNECT was reported true, also `finishConnect`
   will always return true, even if the connection wasn't yet established
Merge pull request #507 from jrudolph/i/437/unresponding-hosts-failure

= io: register for OP_CONNECT only after the connection attempt was made, fixes #437
= can: cosmetic tweaks
= site: update logging config to latest LoggingContext impl
= can: improve reference.conf inline documentation
! can: only render default User-Agent if no such header was explicit
given, fixes #462
Merge pull request #516 from jrudolph/i/462/UserAgent-override-behavior

! can: only render default User-Agent if no such header was explicit given, fixes #462
+ http: add `audio/webm` and `video/webm` media types, closes #518
Added support to play-json with basic error handling.