Implement timer

Add Observable.timer to work below example:

```ruby
require 'rx'

source = RX::Observable.timer(0, 1)
    .time_interval()
    .map() {|x| x.value.to_s + ":" + x.interval.to_s }
    .take(5)

subscription = source.subscribe(
    lambda {|x|
        puts 'Next: ' + x.to_s
    },
    lambda {|err|
        puts 'Error: ' + err
    },
    lambda {
        puts 'Completed'
    })

sleep 5
# => Next: 0:0
# => Next: 1:1000
# => Next: 2:1000
# => Next: 3:1000
# => Next: 4:1000
# => Completed
```