Added a 'zero' option for lines and bars.

Area and bar plots normally use a minimum of zero, since their purpose
is to show size, and using an auto-scaled minimum distorts the plot's
meaning.  But this behavior is undesirable in cases where the plot type
is used in more of a decorative sense.

The zero option provides a way to control this behavior.  It defauls to
true for bars and filled lines.
Update auto-scaling to support the zero option.

Added a format option 'autoscale' that controls whether the given point
is considered when determining an automatic scale.

The lines & bars 'zero' option controls whether autoscale is set on the
dummy point that is inserted to create the series lower-bound.
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.
Merge pull request #911 from dnschnur/series-zero-option

Added a series 'zero' option to control automatic scaling.