Fixed ugly Go variable extraction in Makefiles
Made Status().SetFromString() to return 1 value

Returning the receiver was pointless in this case.
Put back InitBAse + added ZeroStatus() method
First few math functions are in + a few changes

Implemented the first two examples from decNumber as well. By a real
use of the API, I spotted a few flaws.

 - Context.NewNumberFromString() does not return an error anymore. Need
   to check ctx.ErrorStatus().
 - Kept the form ctx.NumberMathOp(result, lhs, rhs) instead of the
   intended res := ctx.NumberMathOp(lhs, rhs). This makes reuse of
   numbers much easier.
New numbers always zero-initialized + doc update
Removed "zero status" from the const and strings

Caused differences in behaviour with C decNumber