Bug 1104013 - Add reftest task for b2g emulator

This the group collection of reftests (crashtest, reftest, and jsreftest). 

The s3 package for xre was added because it's a custom build sdk for emulators that contains a mixture of xulrunner sdk bits and some utilities (such as httpd.js).  Original bug for this zip: https://bugzilla.mozilla.org/show_bug.cgi?id=989143 which is really taken from here: people.mozilla.org/~jgriffin/xre_linux_x86.zip

Caveat: Bug 1104842 -currently no way to separate out test suites for each of the emulator builds because they share the same build task definition.

Taskcluster task:
http://docs.taskcluster.net/tools/task-graph-inspector/#zEG2yskoTg-yQ3xSaBvhFA

mach trygraph command:

mach trygraph --message 'try: -b o -p emulator -u reftest,crashtest,jsreftest -t none' --revision=acde07cb4e4d --repository=http://hg.mozilla.org/mozilla-central --owner=garndt@mozilla.com | taskcluster run-graph --verbose