add complex-valued tests

I've started adding some tests for complex valued functions, to deal with branch cut issues (see #2843).

Currently just has tests for `sqrt`. All tests are taken from the [mac `csqrt` man page](http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/csqrtf.3.html) (it was the most specific).

Some of these tests currently fail (which I guess is the point of doing them). I'm happy to add more functions (log, trig/inverse trig, etc.).