Fix for issue #65: Screenshots not saved into missing folders

Create directory structure if needed when saving screetshots (using mkdirp module).

Includes unit test.
Just realized `mkpath` is already a dependency, so using that instead of `mkdirp`.
Merge branch 'gh-65' of https://github.com/sethmcl/nightwatch into sethmcl-gh-65
updated some tests
added tests for saveScreenshot failure cases
Merge branch 'sethmcl-gh-65'