Fix #24: Suspending and notifying resumption of a download for a media element fetching from a MediaSource

[wolenetz]

@jdsmith3000 Please take a look. I believe this addresses the solution route we agreed upon yesterday for #24 in conjunction with our chat with @travisleithead.

[wolenetz]

Note to reviewers: Please don't use the big green merge button. Instead let me do the necessary rebasing and landing once this looks good to you. Thanks!

[jdsmith3000]

This accurately implements our discussion. I wonder though whether it would be clearer if the reference clarifications to the resource fetch algorithm were bulleted under step 4, like this:

1. Continue the resource fetch algorithm by running the "Perform a potentially CORS-enabled fetch" step, with these clarifications:
2. Text in the resource fetch algorithm that refers to "the download" or "bytes received" refers to data
   passed in via appendBuffer() and appendStream().
3. References to HTTP in the resource fetch algorithm do not apply because the HTMLMediaElement
   does not fetch media data via HTTP when a MediaSource is attached.
4. Text in the resource fetch algorithm that refers to transitions to "NETWORK_IDLE" and firing "suspend"
   events, and the paragraph that begins with "User agents may decide to not download more content at
   any time", do not apply to a media element that has a MediaSource object attached by this algorithm.

Note minor word and punctuation edits.

Numbering above is defined by Markdown, and will be different in the spec. And specifically, step 1 above should be step 4.

[wolenetz]

@jdsmith3000 I like your further formatting and punctuation edits. I'll put those into the PR and merge shortly. My plan is to merge this one, then rebase and merge #86, then rebase, regenerate, fully review and potentially refine further before landing #84. This order allows better review of any HTML5->5.1 changes made by this and #86 before landing #84.