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()`.

Remove form encoded object body.

The body must be consumed through the `text`, `json`, `formData`, or
`blob` functions.

This was based on a misreading of the URLSearchParams portion of
section 5.2 of https://fetch.spec.whatwg.org/#body-mixin.

Use FormData as the request body to POST form fields instead.

Document how to use in Browserify…

IE 9 fails on 204s with error code 1223
http://www.enhanceie.com/ie/bugs.asp

Rewrite the code back to 204. Note: response headers remain unavailable
in this case. This was fixed in IE 10.

Reject all HTTP codes that are not in the 100..599 range.

When an empty reply is received from the server, `xhr.onerror` callback
should get triggered. IE 9, however, triggers the `onload` callback with
status code 12152 that stands for:

    ERROR_HTTP_INVALID_SERVER_RESPONSE
    The server response could not be parsed.

http://support.microsoft.com/kb/193625

IE caches xhr responses pretty aggressively and after restoring the
previous response from cache, it blanks out the "Date" header (probably
by design). The solution for this test is to use a cache-busting query
parameter.

If the body passed to `xhr.send()` is `undefined`, the text sent would
literally be "undefined" on IE.

By default, all static files served from the test server were
"text/plain". IE auto-detects HTML and JavaScript content as such, but
refuses to display the style sheet until it is served as "text/css".

PhantomJS doesn't seem to send request body for PATCH:
https://github.com/ariya/phantomjs/issues/11384

Now only the whitelisted method names get normalized by uppercasing and
the rest are left untouched since HTTP method names in general are
case-sensitive.

In tests we must use `PATCH` (uppercase) because Node.js server doesn't
support `patch` (lowercase).

This fixes PUT/DELETE for IE 9 and PATCH for Node.js.

Document `fetch` usage with Browserify — and switch suggested Promise library to one with a CommoJS API more similar to Fetch.

Auto-detect available port when running headless tests