GeoStore docs and refactor

* `Terraformer.Store` namespace is no longer plural
* `serialize` and `deserialize` methods on stores can now be async
* simplify build process for `Terraformer.GeoStore` by removing the partial
* refactor `RTree.remove` based on the original method
* add tests for `Terraformer.Stores.LocalStorage` when testing in the browser.
* Preliminary documentation for `Terraformer.GeoStore`

All tests pass and the geostore and counties examples both work with no changes.