Add worker code for Link Checker metric

I have added the code for the Link Checker metric. I also noticed that I
have not set the total variable used to report the worker status in the
Accuracy metric.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Create Link Checker metric class with initializer

Similar to the Accuracy metric I have added code for the Link Checker
metric initializer.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add Typhoeus request code for the Link Checker

Just added the code to determine whether an URL is working or not. This
can be done by relying on the response.success? method, as in every
other case either another response code has been delivered or the
request timed out.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Rename Link Checker metric worker

I have renamed the Link Checker metric worker so that it adheres to the
currently used naming convention.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Integrate Link Checker metric into application

I have added the metric to all the files required to launch a Sidekiq
worker process that performs the computations.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add missing initializers for variables

I have forgotten some initializers for variables used in the Link
Checker metric code, as well as a typo.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Fix typo in storing the resource availability

Instead of accessing the nested hash by using the URL I have stored the
boolean directly which leads to erroneous behavior.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add Link Checker property to repository

I have forgotten to add the Link Checker property to the repository
class which is required by the Tire persistence layer. Also added a
case where the score is not a finite float. This commit fixes #4.

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