Make egg_info command write out setup requirements

This commit makes the egg_info command write out setup requirements as well
as install requirements, setup requirements are written to a
setup_requires.txt file.

The commit adds a new function write_setup_requirements which uses the
existing _write_requirements function to write setup requirements out to a file
and adds a new entry point to the egg_info.writers group.
Update docs to reflect new behavior.
Merge pull request #106 from Richard Ipsum.
Update changelog
Update documentation to reflect new function.