Snowplow Java Tracker v0.9.0

Updates OkHttp to OkHttp3 and improves support for Timetstamps. Plus a number of bug fixes!
We've also started publishing our releases to Bintray (https://bintray.com/snowplow/snowplow-maven/) so please update your build configuration to point here for the latest release.

Bump OkHttp to OkHttp3 version 4 (close #175)
Add STM to outbound events (close #169)
Add support for attaching true timestamp to events (close #178)
Update all non-static Loggers to static (close #213)
Fix events sent by example simple-console (close #221)
Alter logging for invalid keys only when adding to TrackerPayload (close #186)
Fix Peru version so vagrant up succeeds (close #216)
Fix Javadoc generation warnings (close #219)