Changed postbuild script and created working install script

Changed the PostBuild.ps1 script to support both Debug and Release output. In the case of release mode it will create a 'dist' folder in the solution root folder, copies all the necessary files, and includes an install.ps1 script that allows for installation of the PowerShell commands. The script creates a folder under Program Files (x86) called OfficeDevPnP\PowerShell\Modules\ etc. and adds the path of the folder to the PSModulePath environment variable (if not already there). 

In the case of a debug output, it will copy the files to a folder in the user's Documents folder to autoload the PowerShell commands instead (no need to modify the PSModuleHome environment variable then).