Fix complex function branch cut behaviour

[simonbyrne]

I've added a bunch of tests (#2845) for the branch cut problems for the functions of complex variables. Currently, some of these tests fail, and so are excluded from make testall (and hence travis), however they can be run manually by make test-complex.

The C99 standard specifies the behaviour of some functions, though is ambiguous for ^ (cpow).

Kahan provides background, as well as some algorithms.

[JeffBezanson]

Have you figured out which functions have problems, or is it all of them?

[simonbyrne]

Yes, it seems that every function raises at least one error.

[JeffBezanson]

I see you did meticulous testing for signed zeros, which is great, but I'm wondering where some of the answers come from. See comments on 3ef1d4d

[ViralBShah]

Perhaps it would be good to also include the source for every test in complex.jl? That by itself will be valuable.

[simonbyrne]

I don't know if it needs to go in the source, but the behaviour should certainly be specified in the documentation.

[JeffBezanson]

@jiahao 's branch merged.