Fix initialization of `Label._text_needs_fitting.`

`_text_needs_fitting` was always initialized to True, even when `max_width` was set to zero. As a result, labels (e.g. plot titles, as shown below) wrap by default:

![chaco_label_bug](https://f.cloud.github.com/assets/133031/1502077/bae6000a-4895-11e3-9f30-a3babc4af8f6.png)

PR Note: The default value for `_text_needs_fitting` doesn't really matter (the change in the conditional does), but it was changed for consistency with the default `max_width`.

Example script:

```python
from enable.api import Component, ComponentEditor
from traits.api import HasTraits, Instance
from traitsui.api import UItem, View

from chaco.api import ArrayPlotData, Plot


class Demo(HasTraits):

    plot = Instance(Component)

    traits_view = View(UItem('plot', editor=ComponentEditor(size=(600, 600))))

    def _plot_default(self):
        pd = ArrayPlotData(x=[0, 1])
        plot = Plot(pd)
        plot.plot('x')
        # Uncomment to trigger text wrapping
        # plot._title._label.max_width = 1
        plot.title = 'multi word title'
        return plot


demo = Demo()


if __name__ == "__main__":
    demo.configure_traits()
```