Revert "add Pask5::Event#{__binding__,created_at}"

This reverts commit 453c2bac68deaf9f9bd77cbe53c06e947c615d7b.
add Pask::Event#__binding__

Pask:Event#binding collides with the (private) binding method implemented
by Ruby. a method with an alternative name, `__binding__` is provided
instead.
add ChangeLog.txt
abort call of Thread#exit on a tracer who has not yet started
rewrite the test suite with minitest/spec
v0.7.0