LSystem initial commit. At this stage it's kind of finished. "Kind of" because
I'm not entirely sure if this will evolve into something else or not. But for
now it's good enough.
m changed name from `LSystem.interation` to `LSystem.step` and related; it's
  just easier to write

+ now rise ValueError when no operations are given but want to execute the
  current string

m modified `LSystem.evolve` to execute at steps relative to the current
  evolution if `execute_at` is given
m changed the layout of the folder and converted it to a module

m renamed base folder from LSystem to procedural and moved files in new folder
  pcg (procedural content generator/generation)
 merged code from bzr project. now this is the main one.