Avoid division by zero

In some cases where diff between max and min can be zero (e.g. when used with
mutable collections where items are deleted), the library will attempt a
division by zero, resulting in this.percentages to contain NaN or Infinity
values.

This adds a check, defaulting to [0, 0, 0].
check for falsy val in setValue)
Change default value for this.percentage[2]. Fix linting.

Set this.percentage[2] to 100 since it's used as a divisor in getPercentage().
When both the dividend and the divisor are 0 in this.getPercentage(), the
result is a NaN value.

This commit also fixes mixed space/tab indentation issues that were breaking
the tests.
Merge pull request #122 from eyko/master

Avoid division by zero