Provide default family and size for text

According to the CSS spec, you can't set the font family and size independently.  That means we need to provide defaults.  I chose "10px sans-serif" as the default, since that is apparently what the [canvas uses](https://developer.mozilla.org/en-US/docs/Drawing_text_using_a_canvas).

This mostly fixes #337.  @attonnnn is probably right that being able to set a global default would make sense; any suggestions on the best approach there?  Something as straightforward as just having a `Crafty.defaultTextSize` property?  Or should it be a property of the component somehow?