Andrewburnell/release changelog

Added in an explicit changelog confirmation for each 'sbt release' to help prevent accidental release of other contributors' code and give a releaser a more sane digest.  This generates a list of all commits since the last release with some metadata and a confirmation prompt.
We should also add notes about the assumptions -- preferably into the README of this plugin -- less likely to be forgotten.  For example that this mechanism assumes that all the tags are v* matching version numbers, and nobody manually tags stuff in the meantime.