Breaking changes

Added the MockXhrRequest class to isolate each call to send() and respond to requests (#30).
Added Node.js package entry points. This is the exports field of package.json.
Changed the call order of callbacks when multiple callbacks exist for the same lifecycle hook.
setRequestTimeout() now throws if timeout === 0.
MockXhrServer.getRequestLog() now returns an array copy.
MockXhr.requestHeaders now returns a copy of the request headers.
Several properties of classes that were meant to be read-only are now protected by getters.
Deprecated MockXhrServer.xhrMock. Use MockXhrServer.MockXhr instead.

New features

Documentation overhaul.
Migrated to TypeScript.
- More accurate typings.
- Type safety caught a few bugs.
Added 'timeout' and 'error' as declarative request handlers for the MockXhrServer.
Use Promise.resolve() instead of setTimeout() to call the onSend hook (#29).
Added MockXhr.getResponseHeadersHash().
Removed noop setters from readOnly attributes.
Updated devDependencies.

Fixed incorrect usage of Blob when calculating the request's body size (#31).
Fixed bug where event handler properties (e.g. onload) were not called in the right order.
Fixed bug where event listener object callbacks did not receive the event as an argument.
Fixed bug where an event listener was called despite being removed by a previous event listener for the same event.