More tests!

While I was playing around with re-implementing the Stream class, I ended up writing a lot of tests. I took some time the other evening to integrate my tests into yours. I also took the liberty of restructuring the tests a bit. If you object to any of this, I can recompose this PR to your liking. Here's what I've done:

- added `test-requirements.txt` to formalize what tools to use for testing
- added `nose` as the test runner of choice
- added `coverage` plugin to get a sense of what might still need tests written
- formalized `pep8` testing
- added new `run_tests.sh` script for local dev, to quickly run `pep8` and `nosetests` in the current python
- added `tox` configuration to test in py26, py27, py34, and pypy
- fixed some py3k incompatibilities uncovered during testing
- moved `tests.py` to `tests/test_stream.py`, removed the execute bit (so nose will discover it). The plan here is to add more test modules for the untested modules, and let nose handle discovery.

Test coverage for `streams.stream` is now 100%, and overall coverage is 91%:

```
$ ./run_tests.sh
......................................................
Name                          Stmts   Miss  Cover   Missing
-----------------------------------------------------------
streams                           2      0   100%
streams.executors                11      3    73%   31-33
streams.executors.executors      19      9    53%   32-33, 36-43
streams.executors.mixins         36      7    81%   43, 64-67, 91-92
streams.iterators                35      0   100%
streams.poolofpools             108     13    88%   63-64, 84, 183-185, 207, 237, 255-259
streams.stream                  185      0   100%
streams.utils                   116     16    86%   90-91, 209-210, 275, 301, 305, 309, 313, 317, 321-325, 335
-----------------------------------------------------------
TOTAL                           512     48    91%
----------------------------------------------------------------------
Ran 54 tests in 0.188s

OK
```