Validate source_date from ABACUS

[egli]

The basic problem is that when ABACUS sends a source_edition which is not of the form "2004" or "blah / 2004" the source_date will be null.

Is this a problem? Well, maybe not. The source_date will be used to generate a bunch of meta data in DTBook, rdf or ncc files for example. A nil value will just generate the empty string.

[egli]

According to http://www.daisy.org/z3986/2005/Z3986-2005.html#PubMed it is optional but should contain a date, so if is empty I guess we should just not emit it.

[egli]

Basically the problem manifests itself in dtb/xml.clj and dtbook.clj. In both places we could make the emit process depend on the value of the meta data. This seems a bit of a sweeping change though.