Add abstract method ->metadata() to MetadataParser.php

[mjordan]

All metadataparser subclasses (MODS metadata parsers, Templated parser, the dc/OaiToDc parser, and the CsvToJson parser) all implement the ->metadata() method in the same way. In the interests of a clean OO API, we should threrefore make that method abstract in the parent MetadataParser.php class.

Running PHPUnit tests with this in place reveals no errors so the change should have no side effects.

Related issue: #400.