Mark FormData support as optional

Conflicts:
	fetch.js

Add promise test helper

Assign to self

Web Workers support

Align used flag error message to Chrome's implementation

Closes #40.

FormData should only able to consume once

Reject promise with TypeError for network failures.

Closes #35.

IE 9+ fixes

See #30 for details.

Previously renamed to _body to detect these types of errors.

Conflicts:
	fetch.js

Move body to _body to prevent direct access

The `body` property may be used for Streams in the future. This also
avoids the temptation to access `response.body` directly rather than
going through a body consumer like `response.text()` or
`response.json()`.