PS + CSOM to enable side-loading in SP2013 (e.g. to deploy apps to a site NOT created with the Developer Site template)..

EnableSideLoading_PSCSOM.ps1

. .\TopOfScript.ps1

[bool]$enable = $true
[bool]$force = $false

# this is the side-loading Feature ID..
$FeatureId = [GUID]("AE3A1339-61F5-4f8f-81A7-ABD2DA956A7D")

# ..and this one is site-scoped, so using $clientContext.Site.Features..
$siteFeatures = $clientContext.Site.Features 
$clientContext.Load($siteFeatures)
$clientContext.ExecuteQuery()

if ($enable)
{
	$siteFeatures.Add($featureId, $force, [Microsoft.SharePoint.Client.FeatureDefinitionScope]::None)
}
else
{
	$siteFeatures.Remove($featureId, $force)
}

try 
{
	$clientContext.ExecuteQuery()
	if ($enable)
	{
		Write-Host "Feature '$FeatureId' successfully activated.."
	}
	else
	{
		Write-Host "Feature '$FeatureId' successfully deactivated.."
	}
}
catch 
{
	Write-Error "An error occurred whilst activating/deactivating the Feature. Error detail: $($_)"
}