Don't double "remove" an item from "dirs"

Use "elif" to only call `dirs.remove(dir)` once if both conditions are true. This is possible if a virtualenv called "test" is present, so both "test/bin/python" exists, and `dir == 'test'`. It would execute `dirs.remove('test')` twice, the second time raising an exception.
util.py: quote failed command for readability
Add Accept: text/html header when processing an index
Merge pull request #1432 from dstufft/set-accept-header

Add Accept: text/html header when processing an index
Vendor pkg_resources and _markerlib from setuptools 2.0.2
Switch to importing the vendored pkg_resources
Remove the setuptools version check

With the vendoring of pkg_resources we no longer need to worry
about the version of the installed setuptools.
Have a nice error when setuptools is not available
Add setuptools bundling to the changelog
Work around python 2.6 not having ``python -m``
Merge pull request #1434 from dstufft/vendor-pkg-resources

Vendor pkg_resources
Use the raw stream to prevent decoding the response
Merge pull request #1435 from dstufft/use-raw-stream

Use the raw stream to prevent decoding the response
Dont' fail on ConnectionErrors
Merge pull request #1437 from dstufft/work-with-dns-errors

Don't fail on ConnectionErrors
updates docs related to using PEP438 options in requirements files
make sure that wheel files with a single digit version do work

this is a rather serious bug, since it makes 'pip wheel ...' fail with
the following error if the wheelhouse contains such a wheel:

,----
| ...
|   File "/home/ralf/.ve/dev/local/lib/python2.7/site-packages/pip/wheel.py", line 420, in __init__
|     self.version = wheel_info.group('ver').replace('_', '-')
| AttributeError: 'NoneType' object has no attribute 'replace'
`----
Upgraded requests to 2.2.0
Merge pull request #1447 from dstufft/upgrade-requests

Upgraded requests to 2.2.0
`get-pip`-first instructions, due to vendoring pkg_resources