Added chocolateyInstall.ps1

When users install our nupkg from Chocolatey, the module will automatically be instaled to $env:ProgramFiles\WindowsPowerShell\Modules\Pester .  For downlevel systems where this isn't part of the default PSModulePath, the machine PSModulePath environment variable will also be updated.
Merge pull request #238 from dlwyatt/ChocolateyInstall

Added chocolateyInstall.ps1