implement using requirements files as dependencies

This resolves #96.
A `Project` exposes two new methods, `depends_on_requirements` and
`build_depends_on_requirements` which can be used to include a
file of type `requirements.txt` in the corresponding dependencies.

Note that runtime requirements will be added to the setup.py script
so that runtime dependencies are pulled in.
fix python3 issue
WTF flake8, this is for py3 compat
some improvements suggested by @aelgru
adding a hash() for python3 compatibility
strip comments from requirements

as explained by @aelgru requirements.txt files may contain comments so
these need to be ignored. Inline comments are not handled right now, I
don't know if these are allowed or not.
rename d -> requirement
refactoring to make @aelgru happy
Merge pull request #102 from pybuilder/feature/requirements-file

Feature/requirements file