Branch tracking

Branch tracking is turned off by default, but can be enabled with the data-attribute `data-cover-flags="branchTracking"`.

With the default reporter, an untouched branch will be highlighted in yellow.

An example can be viewed at `/test/branchTracking/`.

Multi-line branching statements present a problem for the reporter at this time.  This issue still needs to be resolved (in fact, all the branch reporter code could be cleaned up).