Wrapped labels

This changes the space in `Jan 1990` to be non-breaking (`Jan&nbsp;1990`). This eliminates unsightly label wrapping (see issue #482).

Adding this character required giving the axis labels a few more pixels. I changed the default of `pixelsPerLabel` for the x-axis from 60 to 70. This does have a visible effect, but hopefully the change is small and not unwelcome. `Jan 2012` is much clearer than `Jan 12`, which was what we showed before.

This also clears up some oddities around global & per-axis options in the `plugins/axes.js`. Notably `xAxisLabelWidth` and `yAxisLabelWidth` are deprecated in favor of a per-axis `axisLabelWidth`. There was already an `axisLabelWidth` option, but it was only used some of the time in place of `xAxisLabelWidth` :/


Fixes #482