Include all public headers using "soci/" prefix inside SOCI itself.

Harmonize the inclusion style of the headers from inside and outside the
library and always use the "soci/foo.h" form.
Merge pull request #239 from vadz/soci-headers-prefix

Include all public headers using "soci/" prefix inside SOCI itself.
Update version to 4.0.0.

This makes it easier to distinguish the builds using "develop" branch from the
one based on the stable 3.2 branch.