adds features, fixes bugs and refactors

Koike.pm
- new dependency: Math::Trig module
- initial x,y values are 0,0 instead of undef
- allow for alternative color format for svg output. 'rgb' or 'html'
- changes default for 'includehtml' setting to false
- adds support to automatically short-circuit multiple move-to cmds into one.
- adds method 'new_part()' which instantiates and returns a Koike::Part
- adds experimental "directional-color" where the color of a line is set
  according to the direction it follows from start-point to end-point. not
  available for arcs.
- short-circuits svg commands when associated color is 'none'. ie don't add
  invisible line.
- adds 'rb' command to g-code, resulting in prepended perimeter tracing.

Koike/Part.pm
- adds debugging
- replaces the variety of ad-hoc ways to update the current position with an
  internal method call, update_position().
- adds optional boolean argument 'update_startxy', default value true, to
  update_position(). when this method is called internally, false should almost
  certainly be specified.

hexagongrid.pl
- a more or less complete rewrite.

mcescher_lizards_grid.pl
- adds a new file. this is a lot like hexagongrid.pl but creates a logically
  unbounded grid based on M. C. Escher's classic lizards tessellation.