Foliadocserve

Updated tests, compatible with new FLAT v0.9.0, no further functional changes.

* serialise alternative annotations for FLAT (proycon/flat149) (for upcoming flat v0.8)
* fix for alternative annotations
* handling of setless annotations (proycon/flat150)
* fix regression problem upon submission of metadata (proycon/flat155)

* Updated for FoLiA v2, now uses the foliapy library instead of predecessor pynlpl.formats.folia
* Implemented provenance support
* Older FoLiA documents are automatically updated by the new foliadocserve
* Implemented hidden word support (proycon/flat141)
* Allow set definition failures and propagate failed set definitions to output (proycon/flat147)

**Important note:** This release produces only FoLiA v2 documents (it can still read older documents but will automatically upgrade those)

Bugfix release:
* Fixed issue 14 (Metadata changes weren't saved properly if no other annotations were made)

* Bugfix release for metadata changes not saving (13), thanks to OsmanMutlu

* Fixed stop and SIGUSR1 behaviour (12)
* Log file now appends to existing file instea of overrwriting