Updates descriiption and improves regex for safari_lastsession.rb

This updates two things for the safari_lastsession post module:

1. The description is updated: More information is added to describe how Safari would end up storing the Gmail credential in the last session state, and what it means to you as an attacker.

2. Regex update for the domain to search for: Before the module starts extract the session data, it needs to know which domain to extract from. Originally I only added mail.google.com, but turns out the sensitive info can be found in accounts.google.com, so I added that one.