update help to make Package Index Options' usage clearer, ref #1814
Optional finder variable must be validated before use
detect windows when running under IronPython
fixed windows test
Silence byte compile errors when installation succeed.

Fixes #1873.
Cover more cases.
Add show_stderr parameter to call_subprocess.
Revert show_stderr=False to investigate test failures.
Implement site-wide configuration settings

* add site_config_dirs() to appdirs to determine locations across OSes
* add system_config_files to locations.py
* add system_config_files to get_config_files() and re-order files entries to correct precedence
* document changes to configuration files in user guide

Closes #309
Strip tracebacks.
py2 fixes
Merge pull request #1978 from r1chardj0n3s/system-config-file

Implement site-wide configuration settings
another try
describe "non-recursive" as "only if needed recursive"
Merge pull request #1990 from qwcode/only_if_needed_recursive

describe "non-recursive" as "only if needed recursive"
Merge pull request #1984 from berkerpeksag/fix-1873

Silence byte compile errors when installation succeed.
Move unpack_url to pip.download

This puts all unpack functions in the same place.
Cherrypicked from a commit of qwcode
Align unpack_http_url to unpack_file_url
pip.download: add docstring and log
pip.download: refactor and try to clarify

add _check_download_dir and _download_http_url to ease the understanding
of unpack_http_url and unpack_file_url functions