Delegate the implementation of QSMath to <tgmath.h>

[tgmath][1] provides type-generic macros for the functions found in
<math.h> and <complex.h>, so out of the box it handles the branching
logic of QSMath. If relying on a QS-based interface (e.g.,
QSCeil/QSAbs/QSFloor) isn't required, clients could directly import
<tgmath.h>.

All QSMathTests pass.

[1]: http://pubs.opengroup.org/onlinepubs/009696699/basedefs/tgmath.h.html
Merge pull request #11 from andrewsardone/tgmath

Delegate the implementation of QSMath to <tgmath.h>