Convert Store.js module and tests to TypeScript

[kfranqueiro]

Convert src/Store.js to TypeScript as src/Store.ts, and tests/unit/Store.js as tests/unit/Store.ts.

In order to run unit tests with the changes, any instances of Store will need to be changed to Store.default in other JS modules until we convert them as well.

See #130 for general information on the conversion. If you have questions, comments, or thoughts for this module beyond the conversion itself, you can leave comments on this issue.

[kfranqueiro]

This is going to depend on https://github.com/dojo/core/pull/31 being merged, since it uses lang.mixin. I should probably get that out of the way first.

[kfranqueiro]

Comments for future consideration:

• We should reevaluate how much of an opinion Store has on models, and how much it tries to do itself regarding their instantiation/use. I've been on one project so far where we attempted to use dstore with models, and we ended up overriding _restore more often than not. (Also, the use of __proto__ in here has irked me for as long as the code has existed.)
• Either way, we will need to pull out reliance on any declare-specific constructs. (@maier49 will be pulling out the last two Store tests that are currently specific to that, since testing a "declare-like" store had necessitated some awkward code in the test module.)
• (via @maier49 in the PR): What would be more efficient, executing QueryMethod once each time a store is instantiated or executing it each time store.filter is called? I think it's nicer to have this as a method than a property but I don't know if that justifies the extra function calls.
• May want to revisit the tests invoking _setIdentity directly after we convert Memory - we may be able to remove them here without sacrificing coverage, if we test Memory both with/without objects with set methods