Initial version for code review

Because the initial development was occurring on master, there isn't an easy way to do code review. This pull exists solely as a means to code review the entire project. Any changes will be put straight into master and this pull will be closed and the `initial-code-review` branch deleted. All future development will use pull requests into master to facilitate code review.
`getMultiple()` does not return a subset of keys but `getAndSetMultiple()` can if the callback doesn't return values for them. See [my comment](https://github.com/iFixit/Matryoshka/pull/3#discussion_r17387039).