Register builtin extensions via a macro, rather than a manual strcmp

Set the stage for making the builtin modules more dynamic.

Note: this only converts crypto and net, I will add more extensions in a
later commit.

* node.h: Add utility macro for converting macro values to strings.

* node.h: Include the actual module name inside the module structure, not
  just the file it was built from.

* node.h: New Macro, NODE_MODULE_DECL, for declaring an external reference
  to a module structure.

* node_extensions.cc: New File, implements get_builtin_module, which
  iterates over the module structures that are compiled into node.

* node.cc(node::Binding): Use the new module lookup function to find
  modules.

* node_{net,crypto}.c: Add NODE_MODULEs to generate the module structure.