Renamed cmdlets, fixed small issues, eliminated need for entity classes for output

Renamed cmdlets to follow the server cmdlets naming (Uninstall-SPOAppInstance instead of Remove-SPOApp, and Import-SPOAppPackage instead of Add-SPOApp)
Removed all entity classes, they are not needed anymore. Their sole existence was due to PowerShell insisting on trying to resolve unloaded properties and subsequently failing in that, causing the object not to render correctly in the console output. I implemented a format.ps1xml file now and updated the psd1 module file to refer to the format file. 

The new output will be more alike the output of the server PowerShell cmdlets.