Fix hidden return type in custom query. Add for_each_field function

Added missing includes Moved ddl2cpp tests from C++ to CMake

Several fixes for ddl2cpp Several additional data types more portable static_assert / compile tests

Several fixes for MSVC Nicer command line interface for ddl2cpp

Maintenance release, fixing regressions 

Several breaking changes: * `from(a,b)` not allowed anymore, please use explicit joins * `where(true)` not allowed anymore, please use `.unconditionally()` or sqlpp::value(true) * `some_sql_expression and true` not allowed anymore, please use `tab.col == sqlpp::value(true)` if you really want to express this. * `having(expression)` requires `expression` to be made of aggregates, e.g. columns named in `group_by()` or aggregate functions like `count()` or constant values. * `where()` and `having()` accept only one parameter Several more clauses are now available as free functions. Lots of additional tests.

Compiles with MSVC 2015 Update 1 Compiles with Xcode 7 Supports date and datetime data types Supports case when then else Supports empty in() Detect non-aggregate columns in case of group by Lots of small fixes

Added schema qualified tables 

Cleaned up warnings and ambiguities detected by gcc-5.0 

Fixed tvin issues