Overload Pow2 functions to do 64-bit ints

While not immediately useful for MathGeoLib the functions in MathFunc
are convenient for projects that end up depending on MathGeoLib anyway.

In such projects, 64-bit versions of the Pow2 family of functions are
useful.
Merge pull request #25 from zao/pow2-int64

Overload Pow2 functions to do 64-bit ints