Rewrite of the notifications system

It also reintroduces the possibility to turn auto-refresh on and off,
both with the constructor or setter.
Merge branch 'master' into ez-notifications-rewrite

Conflicts:
	realm/src/main/java/io/realm/Realm.java
Add migration methods with old signature
Update javadocs and changelog
Update changelog.txt
Merge pull request #514 from realm/ez-notifications-rewrite

Rewrite of the notifications system