Clean up gradle builds
Add retrolambda to the mix

Retrolambda compiles Java 8 bytecode to Java 7 (only for lambda expressions,
though). That means we can actually write the code that IntelliJ so handily
collapses for us. Interfaces that expose exactly one abstract method should be
annotated with @FunctionalInterface:

http://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html
WIP: Replace loaders with rxjava
Mention retrolambda and rxjava in the README