Feature/eq containment operators

@chifung7 had a pull request (#10) to set the equality scopes to use the containment operator `@>` to take advantage of `GIN` or `GiST` indexing.

However since there have been some divergence in the branches, I applied his changes to the current branch but only for the data types of: `string`, `integer`, `boolean`, `time`, and `date`.

I've also updated the `README` to include examples for `decimal` and `date`.