Start monitoring binary compatibility with sbt-mima-plugin.

We're going to start rigidly enforcing binary compatibility for patch
releases.  Starting with 1.0, we'll enforce it for minor releases.
This is just an advisory integration today.

Fixes #88.
Dynamically calculate the binary compatible release. #88

Rule for the 0.x cycle: if patch version > 0, compare to .0.
Merge pull request #103 from http4s/topic/mima-integration

Start monitoring binary compatibility with sbt-mima-plugin.