Modernize code for Python 3

This is a fairly substantial set of changes, but the main changes are:

1. String attribute issues with Python 3 have all been resolved, and now each function that takes a string can handle unicode strings, and returns unicode strings.
2. All code has been updated to use future imports for unicode_literals and print_function, so we're effectively writing Python 3 code now.
3. PEP8 compliance changes all over the place, except those that would break names required by underlying C DRMAA library.
4. Now automatically run unit tests of Travis-CI with SGE, and all tests pass for Python 2.6, 2.7, and 3.2.  SGE is installed using scripts I describe in [this gist](https://gist.github.com/dan-blanchard/6586533)
5. Unit tests are now in a top-level directory instead of a sub-directory under the drmaa package.
6. There is now a `session.py` module that contains most of the code that was in `__init__.py` before, and `__init__` just imports things and sets `__all__` and `__version__`, as is typically recommended now.
7. Drops support for Python 2.5.
8. Bumps version up to 0.7.
9. Removes `setup.cfg`.

When/if you merge this PR, please update badge URL in `README.rst` to use `drmaa-python/drmaa-python` instead of `dan-blanchard/drmaa-python`. You'll also want to setup a free Travis-CI account and enable building for the main drmaa-python project.