add PWD testing and strict RFC interpretation

This adds test coverage for ```PWD```. 

It also adheres more strictly to RFC959 by rejecting superfluous parameters.

I added ```jsdoc``` documentation to ```PWD```.

Tests cover all execution paths of the ```PWD``` command.

But take note:
- ```PWD``` doesn't enforce what's an acceptable working directory, it just displays the value. CWD will need to ensure that a valid path is set (not ```''```, ```../```, etc.) and that it exists.

Related:
- asylumfunk#15
- asylumfunk#16
- http://usejsdoc.org/
- https://developers.google.com/closure/compiler/docs/js-for-compiler
- https://github.com/jsdoc3/jsdoc