[questions] First attempt at adding Account Observable

This is a rough draft, produced from code, I have been using for a while already.

Note, that unlike most other operators in the library `OnSubscribeAddAccount` is left public to allow people to better adjust it for various needs - the code is already too sophisticated and does too much to try and stuff everything in bunch of static methods.

Before finishing this, I would like to know answers to following questions:

1) How do I test it? Robolectric 2.3 does not have proper shadow for AccountManager, so should I just write my own? By the way, are there plans to update to Robolectric 2.4, perhaps by employing [SDK manager plugin](https://github.com/JakeWharton/sdk-manager-plugin) to fetch dependencies? I am aware of [#1369](https://github.com/robolectric/robolectric/issues/1369), but at current rate it is not going resolve itself.
2) What are project guidelines (ok, what _should_ those be) regarding use of annotations? Complex AccountManager methods need `@Nullable` annotation rather badly, but which one to choose? Android ones? IntelliJ ones (used so far, because they are easiest to get). Perhaps, something more complex (Checker Framework)?
> I'm pretty sure this type is only public because of the builder.

It is public, because attempting to flatten it into multiple static methods would result in ugly API. What would we gain from hiding the operator? Make Javadocs less clear? Neither degree of API exposure nor encapsulation would change - the static methods would still have to do the same thing, just in less flexible way. Besides, there is no "implementation details" being exposed there - just wrapper on top of AccountManager and Observable API.

> Put the builder either nested in AccountObservable or it's own top-level type.

I don't recall ever seeing this kind of abomination anywhere. Top-level non-typed builder? Seriously? If you have to use a builder to work around combinatorial explosion, the problem is already too complex to use static method approach.