Adding collect method to Scala Observable

[madnl]

I think it's a good idea to add a collect method analogous with the one in the Scala collection library. It was simplest to implement it using a combination of filter & map.

If you find any major disadvantages with this please chime in.

[cloudbees-pull-request-builder]

RxJava-pull-requests #329 SUCCESS
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[benjchristensen]

@samuelgruetter can you review this and comment as it related to Scala?

@madnl thanks for getting involved. Once review with Samuel, can you please rebase this on top of the recent changes so it can merge without conflicts again? Right now it conflicts as there was a large set of Scala changes that just got merged to master.

[samuelgruetter]

Yes, adding methods which are in Scala collections but not in RxJava is one of the points which are still on the TODO list.
@madnl your collect looks good, but we would also like to have some unit tests. (We had the following unit test policy for Scala adaptor until now: If a method just calls the corresponding RxJava method, no need for a test, but if new functionality is added, also add a test).

[benjchristensen]

This conflicts so can't be cleanly merged. Can unit tests be added and a new pull request submitted after rebasing changes from master?