Bumped target .NETFramework to version 4.5.1
Added Message model

This model represents en e-mail message
Added Class Diagram

This class diagram illustrates the (intended) structure of the
application.
Added SettingsController class

This class is the main interface for getting and setting application
settings.
Added model representing an e-mail account
Changes to Main Window

Added Tool Strip and Menu Strip, and associated methods
Implementation of Background Worker
Added Folder Tree, Message List and message view
Window for adding a new account
Updated projects resources
Renamed POPClient to POPClientController
Updated project properties
Updated Packages

Removed Microsoft.Bcl
Removed Microsoft.Net.Http
Added SQLite Package
Added Database Controller