Add DB2 features update in doc

Update received from Denis Chapligin.
Work in progress.

[ci skip]
Added a test documenting the current (imperfect) requirements for
using soci::row to retrieve integral values of various types.
Merge branch 'master' of https://github.com/SOCI/soci
One more test, to verify that all string types can be read with
the dynamic binding interface.
MySQL backend documentation update.
Add testing against clang
Add virtual constructor missing from #82

query_transformation_function is pure virtual class
Move table_creator_base subclass closer to test_context