Implement GET API for Elements. Part of #533
correct my copy-paste misdocumentation
Partial implementation of GET API for Element Sets. Part of #533

Doesn't address parameter value of null -- need to double-check that decision when I'm online again, and compare if/how it works for Files.
Use ArrayObject to make sure empty metadata arrays appear as JSON objects.

See https://github.com/omeka/Omeka/issues/533#issuecomment-17413775
Merge branch 'api' of https://github.com/omeka/Omeka into api