feat(publishLast): add publishLast operator

[kwonoj]

closes #883

This PR adds publishLast operator with initial test coverage, mostly mirrored from publish operator's test cases.

[kwonoj]

current implementation of multicast and other publish... does not have selector parameter implementation in RxJS4 (https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/multicast.md#rxobservableprototypemulticastsubject--subjectselector-selector), this PR also doesn't have it yet. (not sure if selector is dropped or not)

[staltz]

Very good question about selector. I know Erik Meijer uses it idiomatically. What does @Blesh @trxcllnt think?

[staltz]

Otherwise, the PR looks perfect.

[benlesh]

The selector in RxJS 4 is meant to make shared things semi reusable by hiding everything you're doing with it in a closure that you can repeat. This isn't really necessary now that you can make reusable connectable observables.

When I had asked Erik about it a long time ago, he told me that he only uses publish() with connect for the most part. Overall, I'm indifferent other than I don't want to introduce it if it's going to result in heavily polymorphic or slow implementations of the operators.

[kwonoj]

Since I personally do not have strong opinions of selector, I will not include those changes in this PR and would like to see if there's demand for those, such as someone might file an issue and allow further discussion if necessary.

[kwonoj]

PR is updated with rebase and add corresponding changes of add auto patching.