Introducing new property for insecure pip install

The property install_dependencies_insecure_install is a list of strings.
Each pip package name in that list will be allowed to be installed
insecurely (may be hosted outside of PyPI, may be unverified).
Note that older versions of pip allow insecure packages by default, and
do not have the required options for a force install.
Introducing backwards compatibility for older pip

If the pip used is too old, and installs packages insecurely by default
anyway, then the command-line options --allow-unverified and
--allow-external will not be used.
Merge pull request #88 from pybuilder/feature/pip_insecure

Feature/pip insecure