Move utility scripts from scancode-toolkit/etc/release to skeleton

[JonoYang]

We use a series of scripts on scancode-toolkit to collect the wheels and aboutfiles of scancode-toolkit dependencies. It would be useful to have this script in the skeleton if we want all nexb projects to fetch their dependencies from thirdparty.aboutcode.org/pypi

[JonoYang]

I'm not sure what is the best way to automate generation of the requirements.txt and requirements-dev.txt files from an uninitialized project.

One process:

1. After configuring is done, if requirements.txt and requirements-dev.txt is not present, then we copy the site-packages directory to a new temporary location
2. We install the dependencies for the gen_requirements.py script
3. We run gen_requirements.py on the copied site-packages directory (This is done so we do not generate a requirements.txt containing the dependencies used by the gen_requirements.py script)
4. We run gen_requirements_dev.py on the resulting requirements.txt and copied site-packages directory

[JonoYang]

@pombredanne suggested that we do not automate requirements.txt and requirements-dev.txt generation. I've updated the README.rst to have instructions on how to generate those files from a new project.