Proposed Fix for Issue #265 - Correcting problem when no parameters passed into Chocolatey-Uninstall

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.  Not sure if this captures all the requirements, for instance, the need for displaying the help documentation, instead, it raised an error.  Would be interested to hear your thoughts on whether this is applicable or not, or what could be done better.  Thanks