Refactor most unit test.

Using Expect() makes them clearer, simpler and more reliable.
Added ability to create and use your own HelpGenerator
Added basic support for overloaded verbs.

Overloaded verbs represent overloaded methods marked with VerbAttribute.
Assuming they can be used with different number of arguments and/or
argument types (as far as C# allows).
Added several other rules of locating the correct verb.
Reorganized code to better fit the task.
Removed useless line of code.
Added nuget.exe
Merge branch 'master' of https://github.com/wernight/CLAP
Merge pull request #20 from Invizory/master

Custom HelpGenerators support
Merge pull request #22 from mikalai-kardash/master

Added ability to select preferred verb instead of the first one.
Auto-formatting and removing some redundant lines
Merging in from main CLAP project following some pull requests being merged.