Add support for raising a `BN` object to the power of a plain JavaScript Number

[barakman]

Currently, the only type of exponentiation supported, is for raising a BN object to the power of a a BN object.

Every other binary operation supports the 2nd operand being both a BN object and a plain JavaScript Number:

• bn1.add(bn2) and bn.addn(num)
• bn1.sub(bn2) and bn.subn(num)
• bn1.mul(bn2) and bn.muln(num)
• bn1.div(bn2) and bn.divn(num)
• bn1.mod(bn2) and bn.modn(num)

So it seems a bit odd that function pow is provided, but function pown is not provided.