Added defined constants for sign.

As far as I can tell the expected values for sign are 1 and -1. I could not see any other supported values in the code. If there are more supported values then please let me know. 

Otherwise this pull request would be useful for two reasons:

 1. It would make it more obvious what values the API expected.
 1. It would help me write my [FFTS Haskell Binding][1].

Thankyou for your time!

 [1]: https://bitbucket.org/robertmassaioli/haffts/src/3300c7ed733e5ee4b460dd699a81c6dea78dd911/src/FFTS.chs?at=master#cl-18