Implement Observable#time_interval
Implement Observable.interval
Add Observable.timer
Fix to work Observable.timer

The context of t.exit is t, Thread.current == t, so the t.exit abort itself.
Add Observable#agregate
Should pass the proc object for Scheduler#schedule_recursive
Fix to work Observable#start_with(1)
Re-fix Observable.zip to be acceptable result_selector block
Add Observable.observable_timer_time_span
Merge pull request #34 from jca02266/fix-zip

Fix Observable.zip to accept a block
Merge pull request #32 from jca02266/implement-timer

Implement timer
Merge pull request #33 from jca02266/implement-aggregate

Implement aggregate
Correct combine_latest same as RxJS's one
Change the Subscriptn to base module
Observable.create block may return Proc object
Add alias RX::Disposable for RX::Subscription
Add dispose method, alias of unsubscribe
Add some examples derived from RxJS observable.md

https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/observable.md
Merge pull request #36 from jca02266/fix-combine_latest

Correct combine_latest same as RxJS's one
Merge pull request #37 from jca02266/add-examples

Add some examples derived from RxJS observable.md