[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)?
The Account creation is is responsibility of an AccountAuthenticator implementation. According to API docs, it must return a Bundle with some values (including account name and type), but there is no mention of immediately creating said account. In fact, until Jellybean it was possible to call most AccountManager methods with manually created `Account` object without actually adding it to system account database. Because of this, the code checks for returned data in addition to fool-proof checks for existence of account. Regardless of chosen implementation, improperly written authenticator still can violate the contract, so in the end the only way to be 100% sure is real-life testing.