Give workers their own ScriptChan and use it for postMessage.

This ensures that XHR callbacks for XHR objects in workers are called on the
worker thread rather than the main thread.
Improve the documentation for ScriptMsg.
Merge pull request #3071 from Ms2ger/workers-threads

Give workers their own ScriptChan and use it for postMessage; r=Manishearth
Change line-height to be read from the font itself, rather than
a hard-coded estimate.
Merge pull request #3074 from glennw/font-line-height

Change line-height to be read from the font itself, rather than a hard-coded estimate.
Make libscript compilation more verbose on Travis.