version to 0.2.0

* Add license to gemspec (7395c6c752f7193e6914f5d438a745606ae23aa2)
* Add query methods (#4)
* Allow no shutdown timeout (#5)
* Change worker-queue interaction, fix race condition (#6)
* Rework `Worker` to use callbacks (#7)
* Modify `Worker`, better shutdown and ignore `nil` work (#8)

/cc @kellyredding