Added Get-SPN Function to PostExploitation.psm1

I think I fixed the things you requested, but if you have any follow up thoughts let me know.  - No rush obviously :) 

**Update Requests Addressed**
- Updated help
- Updated output

**Function Description**
The Get-SPN PowerShell function provides a quick way to search LDAP Service Principal Name (SPN) records for accounts that match a specific user, group, or service type.  This can be handy for finding systems where Domain Admins are logged in during penetration tests.  It's also useful for finding systems were specific services are running.  For example, SQL Servers.  Here is a short blog that provides an overview: https://www.netspi.com/blog/entryid/214/faster-domain-escalation-using-ldap