Add RequiredItemMissing and DuplicateItem exceptions.

NoMetadataError/NoOrganizationsError and their related "too-many" exceptions is too specific,
and we would like to avoid adding those kinds of exceptions every time the standard requires
one, and only one, of some kind of element.

Therefore, introduce the RequiredItemMissing and DuplicateItem, and have the more specific inherit from these
Scorm::Organization should parse the required child <title>
Update the tree binary to use the OrganizationSet
Skeleton for Scorm::Organization::Item
Travis has trouble with Bundler 1.3 on ruby-head, so relax to 1.2. Don't really need 1.3 for anything
Some more things that has to be done