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.
Is it possible to write a test that exercises some function of jrnl? (Create an encrypted journal, write an entry?) cmake is an example of a formula with a good test.