added LogicalBoolean helper.

It would be good to have inside squeryl  common operations on LogicalBoolean-s,  which otherwise need to be written in any nontrivial project with squeryl:

Compose 'and'  and 'or'  statements from list of expressions.
generate true or false value.

So,  I added  object LogicalBoolean  with expression node for constant-false and constant-true logical expressions.