Added an option to avoid exposition of empty collections.
Refactor metadata formats to be smaller

- formats now merely implement an interface
- the relevant element and Item data is passed directly to
  appendMetadata, not set on the format
- formats are registered manually and filtered
Extend DOMElement for convenience methods

Compared to relying on methods on the Generator, this way is smaller and
more natural to write and read.
Remove format dependency on Generator.
Refactor date constants/methods into Date class
Factor out common xsi:schemaLocation handling
Merge pull request #5 from Daniel-KM/empty_collections

Empty collections
Fix notices and reduce get_view calls.
Move new option explanation to consistent location
Get the true earliest date stamp.
Zendified plugin.
Added an option to change the url of the repository.
Returned only public items.
Fixed OAI-PMH request GetRecord.