Restrict zero to bars and filled lines only.

This also includes a tweak to zero's default.  Previously zero only
received a value if lines were visible; now it always receives a value,
matching the behavior of other contextual options.
Mirror the zero option to the categories plugin.

Due to limitations in our plugin architecture, the categories plugin
duplicates code from Flot's core for adding a dummy point to snap the y
axis to zero.  We can get rid of this duplication in 0.9; for now we'll
just update the duplicate to match the change in core that introduced
the new 'zero' option.
Documented the zero option for bars and lines.
Expand documentation of the point format options.
Added the new 'zero' option to the changelog.