Add support for SQL CASE statements

[hisystems]

Add support for the SQL CASE statement. Only TSQL support is required at this stage.

[hisystems]

http://msdn.microsoft.com/en-us/library/ms181765.aspx

Simple CASE expression: 
CASE input_expression
     WHEN when_expression THEN result_expression [ ...n ]
     [ ELSE else_result_expression ]
END
Searched CASE expression:
CASE
     WHEN Boolean_expression THEN result_expression [ ...n ]
     [ ELSE else_result_expression ]
END