Add mvt.php page and hooks in runtest

This commit is the first working version of the multivariate testing
interface.

Changed resultBatch.inc to pull in an optional label field. There was
already (broken) functionality to show the test label in the "Test"
column, but the $test['l'] value was never set.

In runtest.php, the new BuildCustomLabel function can be used in any
context (batch or otherwise) to take a label string template, such as
"{url} from {browser}", and create a label property on the test object
using concrete values: "jrvis.com from Chrome" or "example.com in iOS".
Supported template values so far are: url, location, browser, and
connection. These values are provided in the $labelTemplateMap
associative array, which can be customized to allow any arbitrary
{placeholder} to be replaced with a dynamic value.