Fixed return type deduction for user functions
Fixed return type deduction for user functions

refs #53
The Boost.Preprocessor version of UserFunction lacked definition of
value_type.

Poulpy21/vexcl,kapadia/vexcl,ddemidov/vexcl,Poulpy21/vexcl,Poulpy21/vexcl,kapadia/vexcl,ddemidov/vexcl