fix NameError when initial working_set is empty

Fix for #646 : Wrong error caught in RegistryInfo.lookup

Add test capturing undesirable behavior when unicode characters appear in the filename of a zip sdist. Ref #704.

by defining dist so there's something to delete

only insert eggs ahead of parent if not already on path

ignore .tox in pytest

that includes removing duplicates.

not those loaded by default.

in `insert_on(replace=False)`

and set `replace=False` by default in `activate()`

Fixes pkg_resources modifying sys.path for all eggs
with SETUPTOOLS_SYS_PATH_TECHINQUE=raw

avoids errors in setup.py test if tox has been run

Fix regression in commit e404a4aa

Missing `,`

check if a download is successfull before deciding not to try the nex…

Fix: use os.chdir to change directory (fixes #688)