Observed a strange non-reproducable bug that introduced
'?sessionId=Optional.absent()` (literally) into that pattern.

Largely refactored the code: the individual services are now based on a common
superclass for common tasks, the REST API has moved to a single class,
everything uses the same code for stylesheet caching, cache control,
response headers etc.

We split off stuff that is not HTML specific to an AbstractExporter,
which can later be used as base class for the other exporters.

Added support for the If-Last-Modified HTTP header

Basket functionality is now available for all output methods: Instead of a
single TextGrid URI (referring to a TEI document or aggregation), you can also
specify a number of URIs, separated by commas. The result will contain all
referenced objects, and everything aggregated recursively from them, as well.

Known Limitations
-----------------

The metadata for the overall root object is rather minimalistic. For ZIP export, this only affects the generated filename (which will be <First_title>_etc.zip), for the other content types, this is true, as well.

There has not been much caching optimization for the basket case yet.