Honor pip's -q, --quiet option (issue #35)

See issue #35 on GitHub:
  https://github.com/paylogic/pip-accel/issues/35
Mention the -q, --quiet option in the readme

Should have been part of ba39182d1baebdb6fbc1adcf6b7b9113b82b8fad to
resolve issue #35.
Don't break when download cache contains directories (issue #31)

This fixes issue #31 reported on GitHub:
  https://github.com/paylogic/pip-accel/issues/31
Bump copyright year
Document tox integration in readme (issue #30)

See also issue #30 on GitHub:
  https://github.com/paylogic/pip-accel/issues/30
fixing typo in link formatting
Merge pull request #33: Amazon S3 cache support

See also pull request #33 on GitHub:

  https://github.com/paylogic/pip-accel/pull/33

As discussed in the pull request I definitely see the value of being
able to keep the binary cache in Amazon S3. What I didn't like about the
pull request was that it introduced a call to store_file_into_s3_cache()
inside the module pip_accel.bdist. Conceptually that module has
absolutely nothing to do with Amazon S3 so that had to change :-)

This change set merges pull request #33 but also introduces a new
pluggable cache backend registration mechanism that enables cache
backends to be added without changing pip-accel. This mechanism uses
setuptools' support for custom entry points to discover the relevant
modules and a trivial metaclass to automagically track cache backend
class definitions.

The local binary cache backend and the Amazon S3 cache backend
(introduced in the pull request) have been modified to use the pluggable
registration mechanism. Maybe more backends will follow. We'll see :-)
Encode Python implementation in binary cache filenames

While testing the results of merging pull request #33 I noticed that
when I enabled the Amazon S3 cache backend and ran the test suite using
Tox (which runs the test suite under CPython 2.6, CPython 2.7, CPython
3.4 and PyPy 2.7) either the CPython 2.7 or the PyPy 2.7 tests would
fail. It turns out that the two don't go well together (who would have
thought? ;-). This change set permanently fixes the problem by encoding
the Python implementation in the filenames used to store binary
distribution archives in the binary cache.

See also pull request #33 on GitHub:
  https://github.com/paylogic/pip-accel/pull/33