Support for implicit `$eq`.

This commit adds support for matching scalars with `$eq` implicitly:

~~~js
var doc = { foo: 123, bar: 'abc', baz: true, asd: null };
jc.test(doc, { foo: 123, asd: null }) // => true
jc.test(doc, { asd: 'baz' }) // => false
~~~

It works with scalar values only – strings, numbers, booleans and null. Internally, scalars are eventually normalized to `{ $eq: value }`.

Also included are tests and updates to the docs.