Enable test sharding (pooled environments)

[csnover]

Automatic test sharding per environment across multiple remote instances is a feature that has been requested before. It shouldn’t be too difficult at all to implement:

1. Add a new configuration option for environments that says how many remote browser instances should be started for each environment pool (numShards, numInstances?);
2. Change how test execution works so multiple tests can run in parallel across a pool (the only difficulty here would be flowing across different suites, since the control of test execution is currently delegated to each Suite; the test flow could be moved up into Executor instead, which would also make it easier to implement Feature request: Fail-fast option #413, or a different Suite type could used for the root suites, since each root suite = one environment currently, in the Runner);
3. When each test runs, set the remote property (like in the example which does it indirectly through a test interface) to one from a free pool that is maintained by the test system. Do the same for Suite methods by creating a delegate object (lang.delegate).

[jason0x43]

In combination with #803, this update should allow general test sharding.