Either restore guava or remove use of Optional

Since commit f186971, this project has been using the repackaged Guava classes provided by Jersey. Unfortunately, many classes therin have been ProGuard-ified away, in particular the Optional class. One can see this by dumping the jar contents:

continuity:nater:~/Downloads$ jar tf jersey-guava-2.11.jar  | grep Optional
jersey/repackaged/com/google/common/base/Optional.class
continuity:nater:~/Downloads$ javap -cp jersey-guava-2.11.jar jersey/repackaged/com/google/common/base/Optional
Compiled from "Optional.java"
public abstract class jersey.repackaged.com.google.common.base.Optional<T> implements java.io.Serializable {
  public static <T extends java/lang/Object> jersey.repackaged.com.google.common.base.Optional<T> absent();
  public static <T extends java/lang/Object> jersey.repackaged.com.google.common.base.Optional<T> of(T);
  public static <T extends java/lang/Object> jersey.repackaged.com.google.common.base.Optional<T> fromNullable(T);
  jersey.repackaged.com.google.common.base.Optional();
  public abstract T orNull();
}

Since public interfaces of this package return optional, external consumers can touch these mauled classes and get compilation errors. Either the Guava dependency needs to be re-added (as discussed in #140) or Guava classes cannot be used in public interfaces (or really at all, if they're not in the Jersey repackaged jar).