Add --no-requirements to dh-virtualenv-build

In our case, we are using `from pip.req import parse_requirements` in `setup.py` to put deps in a separate `requirements.txt` but to be required in `setup.py`.

In this case, libs are installed 2 times in virtualenv:
- lib/python2.7
- local/lib/python2.7

I added an options `--no-requirements` to manage this specific use-case.

Based on #60