Copy-GcsObject should support copying to/from local files

[chilversc]

Related to #645, this is the natural name for the equivalent to gsutil cp.

At a minimum this needs better documentation that it only supports bucket to bucket transfers, and should link to the correct commands to use when you want to copy files between the local system and a bucket.

The documentation should highlight how to translate a something like gsutil cp gs://bucke/path/object to the correct commands.

These examples should still support the original URL format so that scripts can be updated to use the powershell cmdlets without needing to change configuration (e.g. a start up script that reads the URL from metadata).