Convert Request.js module and tests to TypeScript

[kfranqueiro]

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

This will also involve replacing usage of Dojo 1's dojo/request with dojo-core's. This should ultimately enable all tests to pass again (right now they're failing due to requestProvider not being configured).

In order to run unit tests with the changes, any instances of Request will need to be changed to Request.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.