BUG: fix #47.
ENH: properly handle egg-info handling for all cases.

We now distinguish between 3 cases:

    - for a non Enthought-built egg, we copy the content of EGG-INFO
      into $site-packages/$package.egg-info
    - for an Enthought-built egg built with egg_info_as_dir=False passed
      to workbench.EggCreator (the default), we copy the EGG-INFO
      content into the correct location inside site-packages, with a
      blacklist to ignore Enthought-specific bits.
    - for an Enthought-built egg built with egg_info_as_dir=True passed
      to workbench.EggCreator, we move the egg-info content into the
      correct location, and don't copy anything from EGG-INFO.
REF: remove unused is_custom_egg in get-dst and write_arcname.
BUG: ensure to remove the written egg-info files.
BUG: don't blacklist EGG-INFO/blacklist in custom eggs.

This is actually a standard feature of upstream egg format.
DOC: mention better egg installation in the Changelog.
Merge branch 'install_upstream_egg_as_setuptools'

Conflicts:
	CHANGELOG.txt
	egginst/tests/test_egginst.py