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'
`----
initial breakup of req.py
Return the native string type
fix req pkg imports
remove req.py
Merge pull request #1461 from dstufft/use-native-string

Return the native string type
fix references to req methods/classes after refactor
use explicit relative imports for Python 3
Merge pull request #1464 from qwcode/req_breakup

break up req.py into a package
Build Python 3.4
Merge pull request #1468 from dstufft/python34

Build Python 3.4
remove unneeded setuptools/pkg_resources checks due to vendoring pkg_resources
have bootstrap install setuptools as well if it's not installed
simpler install instructions due to get-pip.py installing setuptools
Merge pull request #1475 from qwcode/get-pip-setuptools

get-pip.py updates (due to vendoring pkg_resources)
Merge pull request #1445 from schmir/fix-wheel-single-digit-version

make sure that wheel files with a single digit version do work
raise InvalidWheelFilename if version is missing in wheel filename

Wheel('Cython-cp27-none-linux_x86_64.whl') did raise an AttributeError
instead of InvalidWheelFilename.

This has also been discussed an is similar to
https://github.com/pypa/pip/pull/1445
fix typo in test name
Merge pull request #1476 from schmir/require-version-in-wheel-filename

Require version in wheel filename
additional wheel filename tests