Initial six implementation
Attempt at Python 3 support, steps noted below (backwards compatibly not added, but easily done with more time). Tracebacks are failing to show when running lettuce tests, not looked run the unittests on lettuce yet.

repalced "<function>.func_code" with "six.get_function_code(<function>)"
replaced "basestring" with "six.string_types"
replaced ", unicode)" with ", six.text_type)"
replaced "sys.stdout.write(what)" with "sys.stdout.buffer.write(what)"
repalced "six.text_type(" with "six.text_type("
replaced "filter(...)" with "list(filter(...))"
replaced "map(...)" with "list(map(...))"
replaced "reload(...)" with "imp.reload(...)"
imported imp and six where required