Revert "Comment out an optimization in Install.feed()"

This reverts commit 1fd2d87abfaac282a91b03f0d536958b4f602fd0.

The change didn't actually fix the problem. It's actually what was
described before, but the case is different.

The leak happens when a requirement is requested more than once and the
primary requirement is a bare requirement (with no version info). E.g.:

 * gevent            # The order here is important
 * gevent (0.13.6)

If the build process fail happens, curdling will hang.
cli: Error out if the user does not inform a valid command. Closes #40
cli: Fix the list of log levels in the help text
Pass -q to svn in Downloader
Support specifying commit IDs in VCS URLs (Closes #36, #43)

Curdling now supports specifying a commit ID in the VCS URL. The user
can do that by suffixing the URL with a `@` symbol and the revision
ID. E.g.:

  $ curd install git+http://domain.com/project.git@81a57...ca05
Simplify services.downloader.update_url_credentials()
Return Proxy-Authorization header in get_auth_info_from_url() when proxy=True
Support `http_proxy` for downloads we control (Closes #39)

Curdling now reads the `http_proxy` variable from the environment and
uses the `urllib3` proxy mechanism to route all requests through the
informed proxy server;

Authentication is also supported. For that case, inform the user and
password in the proxy URL. E.g.:

  $ http_proxy=http://user:passwd@server:port curd install flask
More tests and small improvements for the downloader
Fix get_auth_info_from_url() for all supported python versions
Manually deal with redirects in the downloader (Closes #45)

The redirects are now handled by the `downloader.http_retrieve()`
function. The number of attempts is hardcoded in the constant
`downloader.REDIRECT_LIMIT`;

The change was made in order to save the contents of the `location`
header, not available in any other way (I could find) in urllib3.
Do not pin urllib3 version on requirements file (Refs #48)

It should make it easier to install curdling on environments with
other compatible versions of urllib3 installed.
Add instructions for running curd-server under docker (Closes #37)

Thanks for the courtesy, @sherzberg!
The Downloader should deal with relative locations in redirects
Relaxed version requirement on setuptools.

0.9.8 is the version bundled with virtualenv, so that's one less
dependency to install !
Merge pull request #49 from madjar/master

Relaxed version requirement on setuptools.
Fixed the makefile to kill the http server with python 3
Relaxed the pined version for wheel.

I tested it with 0.22.0, and it worked
Merge pull request #50 from madjar/master

Relaxed the pined version for wheel
Improve the distribution of threads per service (Refs #35)