Added axis labels

The core text API that we added in 0.8 now supports arbitrarily rotated HTML text on Chrome, IE6+, FF3.5+, Opera 10.5+, and Safari 3.1+.  This was used to add labels to both x and y axes, with the y-axis labels rotated 90 degrees clockwise or counter-clockwise depending on the orientation of the axis.

I also renamed the labelWidth and labelHeight axis options to tickWidth and tickHeight, since all the other tick-label-related options use 'tick' as a prefix, and the old names become confusing when axes now have a separate set of options relating to their label.  The old options are still supported, but are marked deprecated and will be removed in version 1.0.

Since axes now have two kinds of text (ticks and labels) I added separate tickFont and labelFont options to act as individual overrides for the font option.

While making these changes I reorganized and cleaned-up the entire axis options section of the docs.

Note that this doesn't work when using the canvas plugin; it needs to be updated separately to support text rotation.