Introduce spec/run.js

This file builds the test suite for the given environment and then
runs the specs in a child-process. This makes for less duplication
in the Makefile. It also requires reworking `spec/buildSuite.js`
to export a function rather than run directly.
Consolidate Make test targets

The various `test_ember_*` targets didn't produce any output files,
and the tests run fast enough that it's not really worth separating
them, so I regrouped all of them into two buckets:

 * test_stables for a suite of stable releases
 * test_prereleases for testing against prereleased versions
move build scripts to script/
extract script/fetch_vendor.js from Makefile
test against jQuery 2.0.3
relax jQuery dependency to ">= 1.7 <3"

Resolves #92
v1.6.1