Update symbolic links in pip-accel source index before every run
Use package url and version for binary cache file naming
Pin pip-accel to pip < 1.5 (issues #22 and #23)

See issues #22 and #23 on GitHub:

 * --no-install option is deprecated for pip 1.5
   https://github.com/paylogic/pip-accel/issues/22

 * pip 1.5 error - need more than 2 values to unpack error
   https://github.com/paylogic/pip-accel/issues/23
Bug fix for pip_accel.req.Requirement.is_transitive*

* In the previous version, requirements specified in requirements files
  would be counted as transitive dependencies. This was caused by my
  misunderstanding of the pip.req.InstallRequirement.comes_from
  attribute and its possible values:

  - None means the requirement was specified on the command line

  - A pip.req.InstallRequirement object indicates a transitive
    dependency of the requirement specified as comes_from

  - A string like '-r requirements.txt (line 1)' means the
    requirement was specified in a requirements file
Bug fix: pip's --index & --no-index options don't DWIM (now they do ;-)
Bug fix for --index & --no-index issue (dependency links*)

While testing pip-accel on bigger code bases / requirement sets after
implementing the previous bug fix I noticed that it would sometimes
still slow down to a crawl. It turned out that dependency links
specified in setup.py scripts were also being followed. Fortunately the
fix is very simply based on the work done in the previous commit.
Bug fix: Intercept Control-C (KeyboardInterrupt) like other exceptions
Bug fix for issue #24 (regression introduced in 0.11)

When I released pip-accel 0.11 some logic was changed (handling of
requirements with a `satisfied_by' value). This introduced a fairly
subtle bug which I never saw until an issue on GitHub and a complaint
by a colleague pointed out the problem. Here's the issue on GitHub:

  https://github.com/paylogic/pip-accel/issues/24
Merge pull request #21: Use package URL and version for binary cache file naming

See pull request #21 on GitHub:
  https://github.com/paylogic/pip-accel/pull/21