Operation GroupJoin

[akarnokd]

Issue #53

Steps taken:

• Started out from scratch based on the join() operator's pattern
• Consulted with Rx.NET sources to match the data types of left&right maps; and behavior
• Added RefCountSubscription class and consulted Rx.NET on the unsubscription behavior
• Looked at WindowObservable and AddRef in the Rx.NET sources for their behavior.
• Took a test inspiration from http://rxwiki.wikidot.com/101samples#toc39

@headinthebox While writing the test for the operator, I've noticed that when both duration selectors are never(), Rx.NET never calls onCompleted on the group subjects, even though the left and right sources onComplete(). I haven't seen Rx.NET test for this scenario. This comes into play when I want to emulate join() with groupJoin(): the outer merge never onCompletes(). If I understand the behavior corretly, when both left and right sources get depleted, there is no way a new group or new group value would arrive, and all the open groups and the main observer can be onCompleted(). This is somewhat similar to join() and groupByUntil() behavior.

[cloudbees-pull-request-builder]

RxJava-pull-requests #466 SUCCESS
This pull request looks good

[headinthebox]

Thanks man! Things are a bit slow due to Thanksgiving and Ben travelling but I'll try to have a look between the Turkey and the mashed potatoes..

[benjchristensen]

This needs to be rebased as it conflicts with the master branch now.

@headinthebox Are you good with this?

[akarnokd]

Will do after GroupByUntil is merged so I don't have to rebase all the time. Btw, is there a way to add new methods to Observable so it won't conflict all the time? Should I add the new methods randomly in the middle, not at the end?

[benjchristensen]

Typically when I'm adding operators I put them alphabetically inside Observable and the merge/diff behavior seems to handle that better than appending to the end of the file.

[akarnokd]

Will create new PR shortly.