Sets matchers

[lucasfcosta]

Purpose (TL;DR) - mandatory

As discussed on #1210 and #1215, this adds the following matchers for the Set type:

• sinon.match.set(set) - Requires an object to be a Set
• sinon.match.set.deepEquals(set) - Requires a Set to be deep equal another one
• sinon.match.set.contains(set) - Requires a Set to contain each one of the items the given set has.

This also adds tests for each matcher and updates the docs.

Background (Problem in detail)

As it happens with Map elements, the order of elements does not mean anything in a Set and there's no way to know an item's index. This is why this PR does not include startsWith and endsWith matchers.

As we discussed on the PRs linked on the top of this description each one of these matchers work only for Set types and there are tests to ensure this.

The docs follow the same pattern as the docs for Maps and Arrays matchers in order for it to be consistent.

These tests were all ran in IE11 to make sure they would pass.

How to verify - mandatory

1. Check out this branch (see github instructions below)
2. npm install
3. npm run test - This will run the newly created tests

If you desire any further changes please let me know and I'll happily commit them as soon as I can.
Once again, thanks for the awesome job you've been doing ❤️

[coveralls]

Coverage decreased (-0.5%) to 94.906% when pulling 75575b3 on lucasfcosta:sets-matchers into a6d8e14 on sinonjs:master.