Rename function to draw_rule, add documentation and small cleanups
Minor code formatting update
Add comment to LindenmayerSystem class and remove dead code
Add fixed start symbol 'F'

The start symbol of a L-system is fixed to 'F'.
Add parse() function

The function rule_valid() now calls parse(). If parse raises a
SyntaxError exception, parse() return False.
Add building and returning of a parse sequence when calling parse()
Rename function to apply_rules()