Use Math.abs() to ensure number is converted properly.

In some cases, the number is given to the method as a string.  When this
happens, the check for `isNan` and `!isFinite` passes if the string is formatted
like a number, but then number.toFixed() fails because that method doesn't exist
for strings.

By using Math.abs(), we're ensuring the number we're operating on is in fact
`typeof Number`.  It's also a bit clearer to read than the ternary statement
that was previously used, and we don't have to worry and fixing the polarity of
the number once we're done.
Merge pull request #1 from ajbrown/master

Use Math.abs() to ensure number is converted properly.