Add operators to create Observables from BroadcastReceiver (rebased)

[Yarikx]

Rebase #1449 on the top of #1524

Add test for OperatorBroadcastRegister

[cloudbees-pull-request-builder]

RxJava-pull-requests #1449 FAILURE
Looks like there's a problem with this pull request

[dpsm]

LGTM

[benjchristensen]

@mttkay Are you also ready for this to merge?

[mttkay]

👍

[benjchristensen]

Thanks.

[benjchristensen]

Are you okay with this only being released in 0.20, or do you need it back-ported to 0.19?

Have any of you tried 0.20.0-RC3 on Android and confirmed whether it is okay? ==> #1517

[mttkay]

I bumped out beta channel version to 0.20.0.RC-3 today, it will go out
tomorrow to our beta users. I passed all our testing so far, so definitely
nothing major to report. The issues we had were mostly due to race
conditions (which got fixed now) which is quite difficult to reproduce in
automated or even manual testing, so I'll have to wait for logging to
trickle in from the beta release.

[benjchristensen]

Good to know it's passing initial tests. That means the major code paths like merge and observeOn are okay on Android and I didn't make any fundamental mistakes in the sun.misc.Unsafe gates.

There are definitely still some bugs that have been found and fixed but not yet released. Hopefully tomorrow I'll release RC4.