Resolve issues #78, #83 and #84

Mike,
    
This set of changes resolve:

#78: Install modules to %ProgramFiles%\Common Files\Modules 
$env:CommonProgramFiles\Modules is now the default location when -Global is specified.  If $env:CommonProgramFiles\Modules is not in $env:PSModulePath, it will be permanently appended to the machine's PSModulePath environment value

#83: Permanently update PSModulePath environment variable for custom module install paths 
A switch was added to Install-Module, -PersistEnvironment.  When specified, either the User's or Machine's (if -Global also specified) environment variables are permanently updated with the installation location of the module.

#84: PsGet tests should use Pester framework 
All tests have been migrated to use Pester


I have made sure all of the tests pass.  The Run-Tests.ps1 ensures that Pester is installed and imported and will execute all tests that use the *.tests.* naming convention.  

I also updated the Wiki docs here:
https://github.com/MarkRobertJohnson/psget/wiki/How-PsGet-decides-where-to-install-modules


Please let me know if I can provide any other information,

Mark R. Johnson