Add basic 'rake changelog' command

Searches git log for marked commits. Results organized by
categories 'feature', 'improvement', 'bugfix'

Prints results to STDOUT
Only include changelog categories with commits
Add changelog count by category
Include names of all contributors for this release

Don't include names on each entry line. The marked commits may
represent work done by lots of people, so it's not really accurate to
put one name here. Instead include a list of contributors for this
release.
Limit changes search to master branch
Extract changes for category
Treat change log sections as an array
Extract contributors
Remove outdated commented code
Use consistent style
WIP: Add uncategorized commits"
Add miscellaneous (uncategorized) changes
Extract changes code to clarify
Shorten Changelog#generate a bit through extraction
Merge pull request #714 from shoes/changelog

Add task to generate change log entries