Refactor, unicode, Python 3.

Hello!

I'm writting [verboselib](https://github.com/oblalex/verboselib), a little framework for L10N support in stand-alone libraries. Your library is just exactly what I need. But I've found that your current implementation cannot deal with unicode strings and iterator tests do not work on Python 3. Now, if you write 

    StringMock('{0}').format(u'абвгд')

you will get an `UnicodeEncodeError`. So, I propose the next changes:

* support of unicode strings;
* support of Python 3.2 and Python 3.3;
* refactored sources;
* added coverage support;
* `README.md` converted to `README.rst`, `README.txt` was deleted (PyPI works with rst out of box);
* added a couple of badges to readme ([view](https://github.com/oblalex/py_stringlike#stringlike));
* updated gitignore;
* updated manifest;
* added dependency for `six`;
* version update up to 0.3.3.

> Note: I've added badge for `Coveralls`. I think it's a good thing and to make it work, you need to go to [coveralls.io](https://coveralls.io), sign in with GitHub, sync your repos and add `py_stringlike` (just like on Travis CI). Coverage will be automatically updated by Travis. See [my Travis builds](https://travis-ci.org/oblalex/py_stringlike/builds) and [my Coveralls builds](https://coveralls.io/r/oblalex/py_stringlike) for example.
Yes, of course, why not? :smiley:

`0.3.2` is still available at PyPI. I've changed version up to `0.3.3` and you can install package locally, test it and so on. Obviously, you will need to upload new version to PyPI by running 

    python setup.py sdist upload

To test even more, you can play with [TestPyPI](https://wiki.python.org/moin/TestPyPI) also. I'm sure, you know all this stuff :smiley:.

And you still need to implement `__str__` (like in `StringMock`, `LazyString` or in `CachedLazyString` ).

Maybe, it's worth to mention about `text_type` in the docs.

And finally, to make `Coverage` badge to work, you need just to enable `Coveralls` for your project (see 1st message).