262 use hets api

I'm sorry, this is a huge one...

Shall fix #262. Shall also fix #1045 and #516.

I'll try to sum up some points of issue:

- Integrate Hets-Server with rake task and invoker.
- Add IRI-resolving for *standard* iri and versioned iri (with `ontology_version.number`)
- Improve URI-Fetching with a GetCaller class (hopefully)
- Add hets-instances ('Links' to hets-servers)
  - Add mechanic to choose hets-instance (rather crude should be improved later)
  - Add mechanism to retrieve hets-version and to perform a parse-call
- Remove xml- and pp-xml-paths.
- Let parser use an IO-Instance (or a Tempfile, which is technically not an IO)
- Adjust tests.
- Add rake-task to build hets-output fixtures (system call, synchronization of options/command-line-arguments shall be done later)
- Remove dangling old hets-output fixture (with the exception of two, we should try to build them in a new way if we can)
- Change `find_with_iri` to perform a like/postfix search.

--------

What is missing?

- Handling of Hets-Errors (we'll need to recognize the difference between acceptable output and error-output)
- Clever hets-instance choosing
- Web-Interface for managing hets-instances
Lint/UselessAssignment: Useless assignment to variable - `asset`.