Update MessageBox.cfc

Going by this page: http://wiki.coldbox.org/wiki/Plugins:MessageBox.cfm

The function to set a message with "warning" type behaves a bit different than the other 2. You can set a "warning" type msg like this:
```getPlugin("MessageBox").setMessage( "warning", "This is a warning message" );```
OR
```getPlugin("MessageBox").warn( "This is a warning message" );```

To make them consistent with the other 2, i propose to have the same facade:
```getPlugin("MessageBox").warning( "This is a warning message" );```