Added jrnl formula

Hi there, 

I added a new formula for [jrnl](http://maebert.github.io/jrnl/) following your `contributing` guidelines.
It installs on a mavericks host and runs the test correctly.

If possible, please merge.

Thanks for your work.
If I add this line:

`ENV.prepend_create_path "PYTHONPATH", "#{libexec}/python2.7/site-packages"`

The mkdir is not needed anymore?