Build C extension
Use qd-library for double-double arithmetic.

Rather than using `long double`, which is not portable beyond x86 on
Linux and Mac, use the qd-library for double-double arithmetic.  This
has the advantage of greater precision than `long double` as well.  A
bundled version is used by default, or the system copy may be used by
passing `--use-system-qd`.
Add QD library license to the licenses directory.
Update required qd version.
Get working on Mac OS X
Use portable macros for isinf/isnan/isfinite
Merge pull request #17 from mdboom/use-qd-library

Use qd library for double-double arithmetic