client: Move RTC code into rtc.js

Better to have it separated out rather than in the DOM.
Add dependencies for SHA1-hashing of files

The basic idea is that we will have SHA1 hashes of files, and these will be
used as identifiers to request files from peers.

This commit adds the dependencies that are needed to do the hashing:

* FileStreamer.js, for streaming files a few blocks at time.
* sha1.js and sha1_stream.js, which actually implement SHA1.
* worker.js, which does the SHA1 hashing of a file in a web worker.