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 
It's functional -- that's why I wanted your input :)

See changes to dygraph.js. Why do this? It's because the format of some of the labels changed recently from "Jan 08" (in v1.0) to "Jan 2008" (HEAD). I think this is an amazingly positive change, but it does make the labels slightly wider. When the space was a conventional space, this meant that many labels wrapped. (See screenshot in issue #482). With the non-breaking space, it means that the labels get clipped.

So, options are:

  - Increase `axisLabelWidth` but not `pixelsPerLabel`. This won't change the number of ticks you get, but it will mean that there's possibly zero padding between them.
  - Increase both `axisLabelWidth` and `pixelsPerLabel`. This preserves the current padding and the new-style labels, but it is a change.
  - Revert to the old label formatting.

I lean towards increasing both (hence this PR). I'm curious what you see as the downsides of this—it is a behavior change, but I'm not sure it's a harmful one.