Added support for pipelining to Get-Git* cmdlets

I needed to get the status of multiple git projects, but found out that posh-git codlets were woefully unable to be pipelined, so I added few attributes and updated the implementations to support pipelining of `Get-GitDirectory` and `Get-GitStatus` cmdlets.

Also, I started a rudimentary documentation of the cmdlets.

Please take a look at this patch and see if I have not obviously broken anything.