[MRG] Python 3.3 support

[kmike]

All tests pass under Python 2.6, 2.7 and 3.3.

TODO:

• add tests for w3lib.form
• port w3lib.form

There are 2 types of changes:

1. dealing with moved modules and changed builtins: "six" library is used for that;
2. being more explicit about bytes and unicode: a lot of 'b's are added to constants, some tests are duplicated for byte and unicode input.

This PR should be backwards compatible besides the following:

• w3lib.http.basic_auth_header now uses ISO-8859-1 explicitly; previously it implicitly used sys.getdefaultencoding();
• setup.py is converted to setuptools;
• replace_escape_chars unicode handling is fixed.