Adding script to output a project's changelog

[jeremyf]

The changelog.sh script sends to STDOUT commit subjects and authors since the last commit. Additional parameters are available.

[jcoyne]

can skiptag be '[log skip]' so that it's like '[ci skip]' for Travis?

See: http://about.travis-ci.org/docs/user/how-to-skip-a-build/

[jeremyf]

sure thing

[jeremyf]

@jcoyne ready to merge