Optimize visitor implementation.

* The `visit` function will no longer attempt to deep copy nodes when there is an edit. Shallow copies are perfectly acceptable, since we are only overwriting attributes on edit.
* The `Visitor` class now pre-calculates an AST Type to Handler map in the metaclass, allowing for a much quicker enter/leave implementation.
* Install `six`.

Author: jhgg

.travis.yml

@@ -8,7 +8,7 @@ python:
 - pypy
 cache: pip
 install:
-- pip install --cache-dir $HOME/.cache/pip pytest-cov coveralls flake8 import-order gevent==1.1b5
+- pip install --cache-dir $HOME/.cache/pip pytest-cov coveralls flake8 import-order gevent==1.1b5 six>=1.10.0
 - pip install --cache-dir $HOME/.cache/pip pytest>=2.7.3 --upgrade
 - pip install -e .
 script: