Tweak handling of version literals.

* Documentation now gets the version from the same pymssql_version.h
  file as the code.
* Made version scheme more compatible with PEP 386. This means the
  version literal in pymssql_version.h must be of the 'X.Y.Z' form,
  i.e.:

  * No more four-level (X.Y.Z.A) versioning scheme.
  * If e.g. version '2.3' is released the version should be '2.3.0'.

We'll put these notes in a how to release document to be added soon.