Expose simple Item's Broadcasts as a NavigableSet not SortedSet

Simple Items contained a TreeSet of Broadcasts. Expose this as a
NavigableSet rather than SortedSet on Item's interface.

Means that Atlas is Java 1.6+.
Merge pull request #129 from atlasapi/item-broadcasts-navigable-set

Expose simple Item's Broadcasts as a NavigableSet not SortedSet