Prevent unconditional joins, and naked bool in where() or boolean expressions

- `.from(t1, t2)` produces an unconditional join if you forget to add a condition in the .where()
  sqlpp11 therefore now deprecates unconditional joins.
- more often than not, writing  something like `where(name == "doe")`, you
  meant to write `where(t.name == "doe")`. It is hard to find bugs when
  the former expression compiles because you happen to have a variable
  `name` in the current scope as well.
  sqlpp11 therefore now deprecates `.where(bool)` and disallows
  raw bool values boolean expression like `something and bool`
  wrap bools in sqlpp::value(), if you REALLY want a bool value here

tests/DateTime.cpp

@@ -40,7 +40,7 @@ int DateTime(int, char* [])
   {
     std::cout << row.now;
   }
-  for (const auto& row : db(select(all_of(t)).from(t).where(true)))
+  for (const auto& row : db(select(all_of(t)).from(t).unconditionally()))
   {
     std::cout << row.colDayPoint;
     std::cout << row.colTimePoint;