Add Sqrt and SqrtRound functions

[weirdgiraffe]

That is yet another iteration to add Sqrt method into this package. I've already seen previous attempts which were trying to implement the algorithm:

• Create sqrt.go #111
• adding helper functions, decimal numbers and sqrt #112
• Added Sqrt and SqrtRound functionality #130

so this solution doesn't reinvent the wheel and just uses the integer Sqrt provided by big go package. The only thing that this PR functionally adds is the rescaling of integers to fit the desired precision.

This PR just creates couple of aliases SqrtRound = PowWithPrecision(0.5, precision) and Sqrt = SrqrtRound(DivisionPrecision)

[weirdgiraffe]

@mwoss I've seen that you've been involved into reviews of #130, so could you maybe take a look at this one as well?