Wheel support - Initial implementation, DO NOT MERGE

This is an initial implementation of wheel support for virtualenv.

If distlib is available (either installed, or as a wheel in virtualenv_support or any extra search dir) then it will be used to install whichever of pip, distribute and/or setuptools have been requested as part of the initial install.

At present, no wheels are supplied by default - as none of the above projects distribute their code in wheel format, the wheels must be built manually and this is too complex to implement within the current simple ``refresh-support-files.py`` script. (A more complex script could be written, but this has its own risks and issues).

Notes:

* As setuptools does not understand the new ``.dist-info`` format, there are issues using pip and setuptools installed from wheels (possible resolution - require use of distribute if wheels are in use?)
* For anything that cannot be installed from a wheel (which is everything if distlib isn't present) the old code for installing from source and/or easy_install/ez_setup is used. With a fully populated virtualenv_support, this old code will never be needed.
* There are still some questions and issues to resolve - see TODO comments in the code.
* Documentation and tests have yet to be updated.