Use qd library for double-double arithmetic

The `long double` type that is currently used is not portable to platforms other than x86 on Linux and Mac.  This PR uses `double double` arithmetic as supported by the `qd-library` instead.  The bundled version of the library will be used, unless `--use-system-qd` is passed to `setup.py build`.

This requires a small change to `astropy-helpers` here: astropy/astropy-helpers#46