zipkin: fix span duration stats in zipkin realtime
 - fix bugs in span duration stats
 - add tests for monoid

RB_ID=378081
moved Kafka receiver to ThriftSpan

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=391305
Updating install info for zipkin web

Signed-off-by: Jeff Smick <jeff@twitter.com>

RB_ID=391323
fix zipkin-example
sync oss
Fix incorrect parameters to Trace::TraceId.new

 - broken by commit 22a12cff2214e39fe948bd2c1b4a4790e0b5987c
 - also remove extra commas in header names
 - add parsing of flags from header
Allow config hash to be passed on initialization

 - app.config is not always valid except for Rails 3+