Add property page for encouragements

This adds the simplest possible property page for customizing encouragements.

The only problem so far is that you can't hit `Enter` in the text box to add another encouragement. So if you need to add multiple, you have to cut and paste.

The "OK" button seems to steal the Enter keypress. I tried to handle it in the KeyDown event, but I think the OK button receives it first.

@jaredpar any thoughts on what we can do here?
The use of application settings is not version safe in a VSIX. The location of the stored setting file path in part includes the version string and hashes of the executable.  When Visual Studio installs an official update these values change and as a consequence change the setting file path.  Visual Studio itself doesn't support the use of application settings hence it makes no attempt to migrate this file to the new location and all information is essentially lost.  

The supported method of settings is the [`WritableSettingsStore`](http://msdn.microsoft.com/en-us/library/microsoft.visualstudio.settings.writablesettingsstore.aspx).  It's very similar to application settings and easy enough to access via `SVsServiceProvider` 

        public static WritableSettingsStore GetWritableSettingsStore(this SVsServiceProvider vsServiceProvider)
        {
            var shellSettingsManager = new ShellSettingsManager(vsServiceProvider);
            return shellSettingsManager.GetWritableSettingsStore(SettingsScope.UserSettings);
        }

This may or may not be considered a blocker for you.  IIRC the settings just reset on every upgrade so it wouldn't necessarily lose all information, just new information.