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
Revert "Merge pull request #129 from atlasapi/item-broadcasts-navigable-set"

This reverts commit 8745524e0e003571b9fc63a98d479b79aa250ce4, reversing
changes made to ff107714734156fc1bc5f7d261aeca21e58b91ca.
Rename simple Image 'type' field to 'imageType'

Renames simple `Image`'s `type` field to `imageType` and its related
methods to `getImageType` and `setImageType`.
Merge branch 'image-type-field' into stage

* image-type-field:
  Rename simple Image 'type' field to 'imageType'
  Revert "Merge pull request #129 from atlasapi/item-broadcasts-navigable-set"
  Expose simple Item's Broadcasts as a NavigableSet not SortedSet