Migrated to clearer failure return types.

In some cases, this makes debugging with MSVC much simpler, as MSVC does not report the same static_assert as clang and gcc. It gives up when hitting the failure return type (before encountering the static_assert).
The failure return type will now be an accordingly named wrapped static assert.

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