Installation guide should mention that standard "pip install" won't include C extensions

[craigholm]

The current version of pip defaults to installing from wheels, when they're available. This means, by default, "pip install sqlalchemy" will never include C extensions. The installation guide, however, states that C extensions are included unless the platform doesn't support them, and that it shows a warning in this case.

In my opinion it won't be obvious to most users why the C extensions are omitted. The installation guide should mention that pip's "--no-use-wheel" option needs to be specified to install C extensions.

[Mike Bayer]

apparently our rush to use .whl files for publishing on pypi was premature. As we can't place binary built wheels for every distribution, nor can we provide binaries for win32 without external contributions, and we can't place a pure-Python .whl file because it blocks C compilation, they should be removed.

[Mike Bayer]

no more wheels until they have a point fixes #2970

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