Merge tag '0.24' into develop

Re-introduced checks for required columns in insert

Added appropriate constraint tests
Made "where" non-mandatory if there are no tables involved.
Added eval function for both strings and expressions.

eval wraps whatever you give to it into a select call, sends it to the
database and returns the value as a result field.
Fix to make sqlpp11 compile with clang-3.1
Added must_not_update-test
Added alias operators to wrapped value operands
Merge branch 'release/0.25'

Conflicts:
	test_constraints/CMakeLists.txt