Use source-map@^0.5.0 and optimize bundle builds

[zertosh]

This PR should be merged simultaneously with #2424, so there aren't multiple versions of source-map installed. Notable changes include:

• Builds are run through derequire and bundle-collapser. So they're smaller and safe to rebundle (if that's your thing).
• Minified files are compressed and mangled – that's something you have to specify in the CLI.
• Added a minified version of the node bundle. I ran some quick tests and using that instead of babel-core shaves 2 seconds off my builds. But, it does mean that the package then has another 860K to carry around. Feel free to remove this if you want
• Switched all the flags to their long-form so you know what they do. Also turned on bash debugging (set -x) as a poor-man's progress indicator.

before	after	file
2.1M	1.9M	browser.js
1.3M	871K	browser.min.js
12K	12K	external-helpers.js
8.7K	5.7K	external-helpers.min.js
2.0M	1.9M	node.js
n/a	860K	node.min.js
151K	135K	polyfill.js
103K	55K	polyfill.min.js

[zertosh]

Oh, the reason for doing this now is because of mozilla/source-map#205 – they dropped their AMD wrapper thing so running bundle-collapser doesn't break it.