Fix #53, moving a temporary object prevents copy elision

Fix for build and improve forwarding rvalues.

Before change when postpone() with lvalue:
- Total ctor:1
- Total COPY ctor:2
- Total MOVE ctor:2

After: (1 less copy)
- Total ctor:1
- Total COPY ctor:1
- Total MOVE ctor:2

---
Before change when postpone() with rvalue:
- Total ctor:1
- Total COPY ctor:1
- Total MOVE ctor:2

After: (instead copy, move)
- Total ctor:1
- Total COPY ctor:0
- Total MOVE ctor:3

Add listen() alternative

Add Listener::isListening

Requested feature in #48

EventBus version bump v3.0.3

Bug fix

EventBus version bump v3.0.1

Update README

Bump version to 2.6.0

Fix gcc warning

Add AsyncEventBus

Now it is possible to schedule events from different threads.