Simplifications for LambertW

These simplification on LambertW functions are implemented, see (https://code.google.com/p/sympy/issues/detail?id=1981)

exp(LambertW(x)) -> x/LambertW(x)
log(LambertW(x)) -> log(x) - LambertW(x)
Lambert(-pi/2) -> I*pi/2