Added support for filesystem sanity checking

With a clean git clone, the unit test on the ```save()``` method when attempting to write a new entry with photo. This is because git ignores empty folders such as ```/test/data/photos/```. When the unit test attempts to write to that folder, it's unsuccessful because it didn't test to see if that directory existed.

Added these checks to this case, and other locations where we would want to sanity check the path generated before attempting writes.

Also added an assertion to the DayOne.js unit tests that was hiding this bug in the ```remove()``` unit test.

@pengwynn Might I trade this for an atom.io invite? :smiley: