Fix: Duplicate Device Models

Stop future duplicate device models and repair any existing duplicates
SEE:
http://discoict.com.au/forum/support/2013/2/duplicate-device-models.aspx
Feature: AD Update Last Network Logon Date

Ability to run this task on-demand from the web UI is added
Rename: WirelessCertificates -> DeviceCertificates

Code updated to reflect change from 'WirelessCertificates' to
'DeviceCertificates'
Update: New Plugin Framework

Plugin Framework re-written. Initial commit.