Implement LocationMarker object type.

This is the master-branch patch analogous to #185, which applies to
the harmony branch.

https://code.google.com/p/esprima/issues/detail?id=429
AMD test: Do not load via script tag explicitly.

The loading of Esprima should be handled by RequireJS.

http://code.google.com/p/esprima/issues/detail?id=291