Adjust unit tests to the metric renaming

I forgot the spec folder when I grepped for the old metric names. Thus,
the code I have pushed broke the build. This fixes it again.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Overhaul eager loading of metric subclasses

By introducing library modules that get included in the metric code I
ran into a loading dependency problem in Rails. Since I was not able
to resolve the loading order in a conflict-free way.

The new solutions drops the inherited method approach and creates the
list of metrics based on the directory content. Although I do not like
this approach I had to iterate the directory's content anyway. So it is
more or less the same solution with less code.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Overhaul metric class loading once more

Eventually, I have found a more sound solution to the metric class
loading. Still, the metric class loading has to be a vital part. I
changed the configuration so that the lib directory should only be
loaded once. Doing so conflicting dependencies in the reloading have
been eliminated.

In addition, I have added a config.to_prepare block. This block is
executed once at boot-up and in development mode also before every
request. This solves exactly my problem in which Rails removes all the
classes again if they have been modified but did not reload them again.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix a series of problems with the accuracy metric

The accuracy metric has been improved for a number of open issues. The
resource size is measured again and I was able to eliminate some
problems with the URL encoding.

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