Add an example of Observable.if
Implement the Observable.if
Add an example of Observable.merge
Implement the Observable#pluck
Add merge that is alias of merge_all
Fix to work merge_all using default value of scheduler
fix to work examples/merge

There is a bug.
when RX::Observable.merge(...).take(5), threads of interval do not exit.
Add an example of Observable#merge_all
Add an example of Observable.never
Add an example of Observable.of
Implement the Observable.of
Move lib/rx/linq/fork_join.rb to lib/rx/linq/observable