Another attempt to improve project structure

I have undone the whole markdown thing, now it's just:

* Adding a 2 requirements files:
 * `requirements.txt` contains the dependencies of diesel itself
 * `requirements-dev.txt` contains the dependencies for hacking on diesel (mostly testing tools)

I have used [pip-dump](https://github.com/nvie/pip-tools#pip-dump), from [pip-tools](https://github.com/nvie/pip-tools) to generate the requirements, and because I added `requirements-dev.txt`, if you update any testing dependency, all you need to do in order to update the requirements is run `pip-dump`.


* `setup.py` was improved to read the dependencies from `requirements.txt`

* Improved `Makefile` to use nose along with coverage and rednose:
 * [rednose](https://pypi.python.org/pypi/rednose/) improves the error feedbeck a lot when running tests
 * coverage helps us keeping track of how much more coverage is required in the code (that's how I get myself focused to cover the codebase with tests)

Last but not least, I integrated diesel with travis support but I wasn't abble to trigger it, I guess it's because your upstream repository doesn't hook into travis yet. Would you mind enabling it?