storage: support resumable uploads

## Do not merge yet - still writing tests

RE: #298

```js
/*! Developer Documentation
 *
 * `createWriteStream` uses the Resumable Upload API: http://goo.gl/jb0e9D.
 *
 * The process involves these steps:
 *
 *   1. POST the file's metadata. We get a resumable upload URI back, then cache
 *      it with ConfigStore.
 *   2. PUT data to that URI with a Content-Range header noting what position
 *      the data is beginning from. We also cache, at most, the first 16 bytes
 *      of the data being uploaded.
 *   3. Delete the ConfigStore cache after the upload completes.
 *
 * If the initial upload operation is interrupted, the next time the user
 * uploads the file, these steps occur:
 *
 *   1. Detect the presence of a cached URI in ConfigStore.
 *   2. Make an empty PUT request to that URI to get the last byte written to
 *      the remote file.
 *   3. PUT data to the URI starting from the first byte after the last byte
 *      returned from the call above.
 *
 * If the user tries to upload entirely different data to the remote file:
 *
 *   1. -- same as above --
 *   2. -- same as above --
 *   3. -- same as above --
 *   4. Compare the first chunk of the new data with the chunk in cache. If it's
 *      different, start a new resumable upload (Step 1 of the first example).
 */
```
It's actually the number of bytes that have already been written by a previous upload attempt. So, if I'm resuming an upload after it was uploaded half way, this variable would come to hold basically the index of where we left off -- the number of bytes that have been written so far.

Is there a better name out there you can think of?