Ability to follow --requrement in spec files

Now pip-compile is able to handle dependencies between spec files.
For example, usually I have all basic requirements in 'base.in'
file and additional packages, useful for development - in 'develop.in':

    # base.in
    django

    # develop.in
    -r base.in
    pdbpp
    django-extensions

Now `pip-compile develop.in`, will pull all necessary dependencies
from `base.in` file and resulting `develop.txt` will contain all
necessary information.