more methods for real, imag

[jverzani]

Add methods for real, imag.

Doesn't quite fix #273 which raises the point that a symbolic value is treated as real here, but there is no way of creating a Complex symbolic value directly with complex or even Complex, as they expect isa(x,Real) to be true. Perhaps a new method

Base.complex(r::Basic,i::Basic) = r + i*IM

(with some promotion) would be useful.