Enable running pip-accel as a module (fixes #69)

Refer to issue 69 on GitHub for additional details:
  #69

Enable Boto workaround only on Python 3.x (issue #68)

Refer to issue 68 on GitHub for details:
  #68

Remove spaces from install_requires specs*

Just ran into this obscure error message from `python setup.py egg_info'
that I've never run into before today:

  error in pip-accel setup command: 'install_requires' must be a string
  or list of strings containing valid project/version requirement
  specifiers; Invalid requirement, parse error at "', < 7.2'"

I don't quite grasp how I've never run into this before with similar
versions of setuptools, however the fix seems simple enough and harmless
so I thought I'd publish the fix anyway.

Improve confusing installation messages*

Due to the way the TransactionalUpdate context manager was being used, the
uninstallation of a conflicting installation would be logged and performed
before the package's installation was even announced. The resulting order
of (un)installation messages was a bit confusing (even to me :-).

Bug fix: Avoid version=None when upgrading packages*

Refer to the comment in TransactionalUpdate.__enter__() for more details.
The end result was that InstallRequirement.version would be None and so
the source distribution would be cached under the version None,
potentially causing all sorts of issues.

Really cleanup `.eggs' symbolic links (related to #49)

Cleanup `.eggs' symbolic links when no longer needed (improves #49)

Proper support for "user scheme" installations

Experimental support for "user scheme" installation

Enable caching of setup requirements (fixes #49)