Allow unary negation to work with all distributions

[venpopov]

Summary

Fixed #95. I added a test showing that -dist_wrap('norm') and -dist_student_t(3) fail before the fix and pass afterwards.

Not sure if this is the optimal approach, but I just copy pasted the part from Ops.dist_normal that handled the unary negation to the code for Ops.dist_default and Ops.dist_transformed

[mitchelloharawild]

Looks great, thanks!