release 0.3

many bugfixes and code cleanup, and a bunch of new features added.

* now 'sh configure' before 'make' (which is a good thing)
* boolean attributes also accept "true"
* layer attributes that use columns can also use constants
* <points> attribute "fill" to draw filled
* more functions for <rpn>
* numeric layer attributes can also take column names
(<line width> <points radius> <segments width>)
* four new <image> attributes controlling background and border
* string width modifier now supported in more cases of <label> templates
* <area> and <segments> attributes "x0" and "x1" secondarily default to "x"
* <area> and <segments> attributes "y0" and "y1" secondarily default to "y"

* fixed bug 598085
* don't draw <points> with invalid radii
* fixed a bug where not all units where supported
* made it so that an axis that only displays one value (min==max)
auto-adjusts so that the value appears in the center of the axis
* fixed bug where it would segfault with an empty <label> template

* refactored how data is pulled out of a column
* more code cleanup and commenting
* safety code in calculating <label> ticks if more ticks than pixels
* optimized memory usage a little