Add `_slice` method to abstract array conversion

This commit adds a private `_slice` method to the Postpone class to
convert iterable objects into arrays. This method performs the conversion
by trying to call `Array.prototype.slice` on the provided object and then
defaulting to a manual iteration if the first method fails.
Continue adding support for IE8

This commit abstracts some methods inside of Postpone so that the
execution of the methods is agnostic to their implementation and they
can be implemented to support non-standards-compliant browsers.
Correct IE8 event name.
Finish IE8 enhancements.
Merge branch 'master' of github.com:lsvx/postpone
Update documentation and bump versions.