Spksrc: Replace pip bundle with pip wheel. 

First implementation of replacing bundles with wheels. 
`Note: Pulling this is fine, but be careful with installing the examples on your Synology.`

From http://www.pip-installer.org/en/latest/cookbook.html: “Wheel” is a built, archive format that can greatly speed installation compared to building and installing from source archives. For more information, see http://wheel.readthedocs.org/en/latest/.

Spksrc will create wheels when the `WHEEL` variable is specified in `Makefile`.
Copying the wheel folder into the package.tgz is done in `_extra_install`.
After creation of the virtual environment, the wheels can be installed in a similar fashion to the pybundles.

Issue: In some cases (e.g. PyYAML), a platform-specific wheel is generated for some reason. These cannot be used. Creating wheels for other arches does not seem possible...

Three examples are added: Subliminal (working), NZBConfig (working), Flexget (not working/PyYAML).

Ref https://github.com/SynoCommunity/spksrc/issues/184#issuecomment-30331710