Port selector matching to use atoms.

Removed the Lowercase new type. This does mean that this
code path is slightly slower for now. This is because
find_equiv() doesn't work with atoms in hash tables, due to
the hash value being different than that of a string. However
it also means the removal of some unsafe code, and a better
long term solution will be to precache the lower case versions
as atoms.
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.
Merge commit 'refs/pull/3029/head' of https://github.com/servo/servo

Conflicts:
	src/components/style/selector_matching.rs
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.
Merge pull request #3072 from Ms2ger/libscript-log-travis

Make libscript compilation more verbose on Travis; r=Manishearth+SimonSapin