(chocolatey-community#58) Speed up Get-RemoteChecksum

The current Get_RemoteChecksum implementation uses Invoke-WebRequest
which can be slow on larger files. This results in a long checksumming
process during automated packaging. By switching to the .NET webclient,
while losing the "interactive" download update, we significantly increase
the speed of the download. For example, on a 150 mb file, it went from minutes
to download to seconds.

src/Public/Get-RemoteChecksum.ps1

@@ -8,7 +8,8 @@
 #>
 function Get-RemoteChecksum( [string] $Url, $Algorithm='sha256', $Headers ) {
  $fn = [System.IO.Path]::GetTempFileName()
- Invoke-WebRequest $Url -OutFile $fn -UseBasicParsing -Headers $Headers
+ $wc = New-Object net.webclient
+ $wc.DownloadFile($Url, $fn)
  $res = Get-FileHash $fn -Algorithm $Algorithm | ForEach-Object Hash
  Remove-Item $fn -ea ignore
  return $res.ToLower()