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.

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I've tested as much as I could locally, now submitting this PR to see if Travis finds any problems I missed.

The changes here are pretty trivial but they were done automatically (i.e. using sed), and there are a lot of them, so it's not impossible that I did break something.