Fix brittle catalog test

The old test actually made an HTTP request to an external service.
The new test stubs (only) the subprocess call of hets with a catalog.
use and_return.
remove stub calling the original.
comment the use of any_args.
Merge pull request #1076 from ontohub/fix-catalog_test

Fix brittle catalog test
add start/stop rake task for hets server.
support invoker for managing hets.
add mime-router and grouped constraints

MIME-Router will allow to filter based on mime-accepts-header
and GroupedConstraint will allow to include multiple Constraint
in an 'and' fashion (all constraints need to be return true).
handle iri for html and plain

In the html-case just perform show action.
In the plain case send the ontology-file as download.
replace mixed iri/download link two links

The iri link (which displays the iri of an ontology)
now actually links to the iri. And the download-link
has moved as a button next to the evaluate button.
add basic classes to call hets-api.
add mechanism to handle unfollowableness

Add a custom errorclass to uri-fetcher in order to detect and handle
unfollowable responses.  Unfollowable Response in our case actually
means two things

1. the response does not have a location header set, which would be
needed in order to follow the response to a different location.
2. the response itself is not a response which yields a body suitable
for us. suitableness is determined through the overrideable
  has_actual_content? method.
create hets-instance model

This also includes creating the default localhost
hets-instance during the seeds.

Additionally some convenience methods for the version
of a hets-instance have been created.
add hets-version-caller into the mix

The VersionCaller can retrieve the version of
a running hets-instance by calling the restful
API. It also serves as a sort of heartbeat in order
to determine whether the hets-instance is
reachable (internally called 'up').
add specs for hets-instance functionality.
add missing webmock to rspec

In order to mock web-requests we could/should
use webmock instead of regular method-stubbing with
rspec.
add functionality to determine up-state for hets

Before save-actions occur we should perform
a reachability test. This will allow us to
check whether a hets-instance is 'up' and
to retrieve the version which it is using.
move hets errors into own module and add new error

Add the new error which is to be raised if a hets-instance is supposed
to be used, but not actually reachable/up.
raise HetsInactiveInstanceError if appropriate.
increase hets-minimal-version and add key

Add key for the general version of hets.  Currently not really important
for version checking purposes, as the timestamp is thorough enough, we
  will need it for better validation.

Necessary changes in hets: wget-headers for ontohub-iri file retrieval.