fix FLOOR behavior for values of "significance" that are not powers o…

[patriciali]

…f 10

#63 fixed a bug in CEILING. I realized FLOOR has the same problem. this PR fixes it

some code cleanliness changes, in addition:

• CEILING implementation changes
  • move the Math.abs(significance) call to after we've actually parsed it as a number and checked for errors
  • use mode = (mode === undefined) ? 0 : mode instead of mode || 0 since checking truthiness of numbers is kind of an antipattern
• FLOOR implementation changes
  • remove the check !(number > 0 && significance > 0) && !(number < 0 && significance < 0) for consistency for CEILING, which just silently takes the absolute value of significance
  • use exports.FLOOR.MATH = exports.FLOOR to be consistent with CEILING.MATH
  • remove mode === undefined check after we've already parsed it as number
• change the ordering of FLOOR/FLOOR.MATH/FLOOR.PRECISE tests to be consistent with the analogous CEILING tests