Start to write unit test for the archiver class

As a matter of fact, I am starting to write now a lot of unit tests for
both projects, Metadata Census and Metadata Harvester. Here I start with
the archiver class as I want to replace Yajl with Oj. So far the
archiver's constructor is tested.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Write unit test for Archive#store and replace Yajl

I have written the specification for the store method. Doing so I have
also replaced the Yajl parser with the Oj parser. It all works nicely.

Interestingly, the Oj parser conserves Ruby classes like symbols or the
Date class. I have adjusted the header hash to make sure it is all
strings and numbers.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix conflicting relative paths

The relative paths are quite dangerous. For the unit tests I ignore that
for now as I will not expect the tests to be called other than by the
command rspec in the project directory. Also made sufficient changes to
get the API harvester working, too.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add missing code to complete the JSON dumping

Due to some local reverts I have made these changes got lost. Here I add
them again. This should make the JSON dumping complete again.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>