Replace instances of 'hash' with 'object' in documentation

Context: https://twitter.com/mvirkkunen/status/506796515982311424

Many of us programmers know what "hashes" are in the general sense, but the word is not part of the JavaScript vernacular and could be unfamiliar to newcomers. This PR replaces instances of "hash" with "object" in various markdown documents.

This is a "map" or "mapping" – simply calling it an object does not capture what it's doing or why it exists.