Linux Gather 802-11-Wireless Security Credentials

  This module collects 802-11-Wireless-Security credentials such as
  Access-Point name and Pre-Shared-Key from your target Linux CLIENT
  machine using /etc/NetworkManager/system-connections/ files.

  In most of the current Linux distributions, `NetworkManager` keeps
  "psk" in plaintext format (by default!) and the module collects this
  information.

  Gathered credentials are saved in the csv format.
  (Tested with Ubuntu 14.04 and other Ubuntu-based distributions)

![screenshot](https://cloud.githubusercontent.com/assets/8256314/4090496/780c224e-2f7d-11e4-983c-1210f7626c34.png)

The changes I added to msf/core/post/file in #3599 should be useful here.