Implement a LocationMarker object type.

This change gives a noticeable performance improvement on the harmony branch, when either the "loc" or the "range" parsing options are enabled.

Note that test/benchmarks.js does not enable either of these options, and so those benchmarks don't show any improvement with this patch applied.

When I run `node test/runner.js`, I see a consistent 38% decrease in running time (from ~1100ms to ~680ms on my machine).

When I run the same test suite in Chrome 28, the running time drops from ~1700ms to ~1200ms (a 30% decrease).

When I run the test suite in Firefox 21, the running time drops from ~2300ms to ~2000ms (a 13% decrease).

The performance win is attributable to inheriting the end, applyGroup, and apply methods from LocationMarker.prototype instead of redefining them for each instance.