Externals refactored.

Externals refactored.

Public API is rectified to allow the creation of external objects in
runtime.
    1) njs_vm_external_add() is replaced with njs_vm_external_prototype().
    The later functions returns a pointer to a prototype object which can
    be used to create a value with such a prototype in runtime.

    2) njs_vm_external() is split into njs_vm_external_create() and
    njs_vm_external_bind(). The former creates a variable with a specified
    prototype and associates it with an external pointer. The latter binds
    a variable to a name in the global namespace.