Issue #6: JSR 305 integration.

First pass at documenting null safety and thread safety using JSR 305.

 * Put a `package-info.java` in every package with a `@javax.annotation.ParametersAreNonnullByDefault` annotation. 
 * Used `@Nullable` to document the various and places where nulls were being passed.  Have not yet documented any sort of return value. 
 * Documented the `com.readytalk.revori` package with a best-guess as to what level of thread safety was intended/whether the classes are supposed to be immutable. 
 * Cleaned up the headers so that the code is a little cleaner.