* Minor documentation tweaks and corrections.
* Grammar in evo.ge.Ge is always transformed into the evolutils.gramar.Grammar as it is needed by the subtree mutation.
* Mutations are set up similarly like crossover using the setup_mutation() method.
* Changed annotations from subtree crossover are stored in the offsprings.
* Introduced subrree mutation, variable mutation.
* evo.utils.grammar.Grammar.to_tree() now can take extra argument allowing to override the start symbol.
* Annotations returned from evo.utils.grammar.Grammar.to_tree() are now lists instead of tuples.