Update Install-ChocolateyPackage to check for cached package installers

The `Install-ChocolateyPackage` helper downloads package installers to a local temp directory. This patch adds a quick check to see if the file exists in the temp directory before downloading it again. This changes the temp file name to include the remote file name to help prevent name collisions between different versions of installers.

In this pull request, I also added the same local cache resolution strategy to the `Install-ChocolateyZipPackage` helper.

Some installers are rather large, and saving them helps save time and bandwidth, and possibly working towards a complete offline-accessible mode as discussed in Hanselman's blog post on working with NuGet offline.

This is my first Chocolatey pull request, so please let me know if there's anything else I should do to meet the project standards. Thanks for your patience from the other pull request.