Add feature allowBlank, document code, add tests, add teardown

Hi! I am using ember-datepicker in a private project but found myself missing the ability to allow the picker to have a `null` date to indicate an error in my form validation.

Digging into the source I found it widely undocumented (which is not too bad since it's not so many LOCs), but trying to figure out how it works, I added some documentation. Also, hoping I won't break anything I noticed there weren't any tests, so I added some that cover some basic functionality.

I hope you find the `allowBlank` feature worth the hassle of merging this, and agree on the general direction with comments and tests I went. Also, turning on travis-ci is a no-brainer, it really works out of the box! When you merge this, please adjust the URL of the "Build passing" image so it points to the right repository.

If you have any questions, please get back to me,
Stefan