Export global Exoskeleton variable too.
Add utils.ajax beforeSend option.
Use only _ methods which exist (for custom lodash builds).
Optimize events for no-args case `.off()`.

This case is often used in frameworks like Chaplin which unbind all events on model disposal.

@jdalton @akre may be curious that this increases performance by three times http://jsperf.com/exoskeleton-vs-backbone-events
Optimise collection and model methods for custom lodash builds.