Implementation of a new Quad-double type for Boost.Multiprecision

A new quad-double type would be implemented, initially as a wrapper around the QD package's qd_real class authored by David Bailey. Tests for correctness and benchmarks will be subsequently written along with the documentation. If time permits and if the mentors find it feasible, a new multiple-double type based on the same algorithms as QD package will be written with no dependency on external libraries.