re-doing change in version 1.4

I was a noob and royally hosed my fork when trying to bring it in line with 1.4.  So, I blew it away and started over.  As such, I've closed pull request #123 and I'm re-submitting this (hopefully clean) one.

This adds an option to the spork bump plug-in to prompt the user for a comment about their change (the reason for the bump). This comment is appended to CHANGELOG.md in the following format:

<new_version>
--------
<user> - <comment>

The Git plugin was also updated to 'git add CHANGELOG.md' during ":after_bump"

Future ideas/ possible refinements:
  - Pass comment to a git commit.
  - Pass comment to various communications plugins (we don't currently use any of these, so may need help here).
  - Pull windows username as well as UNIX "ENV['USER']"