Warnings when using deprecated :overflow_policy thread pool option

* Thread pools still accept the `:overflow_policy` option but display a warning
* Updated changelog with post-0.7.1 updates

Finished work began in PRs #199 and #201. Completes enhancement suggested in issue #192.
Ah - this one wasn't something I accidentally missed. It's a test to make sure we're backwards-compatible with `:overflow_policy`.