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)

docs/easy_install.txt: remove repeated word.

specify that no 'working' links were found, previous behaviour didn't check if downloads could succeed

docs/conf.py: remove unused boilerplate.

CHANGES: make exactly one space after period.