Proposed fix for #265 - Added check for packageName and added Unit Test to cover work

In an attempt to help out, I have had a stab at fixing the reported
issue regarding calling Chocolatey-Uninstall with no parameters.  I have
added what I "think" is the necessary code, along with a test which
proves that at least it does what I think it does.  Let me know if you
have any questions.