Implement 1st and 3rd Quartile

[e-kwsm]

Currently this library provides only median; it is convenient if quartiles are also available.

[e-kwsm]

I copied the codes for median and modified a little so the patch is boilerplate, though.

[ericniebler]

Thanks for the patch. I currently traveling to the C++ standardization committee meeting in Kona. I'll look at this as time allows.

[breese]

I think that this is already covered by extended_p_square