Delegate Metrics module code into a separate file

I faced some problem when trying to remove some redundancy. I wanted
put the metrics identifier in one place and one place only. With this
I also wanted to generate the repository properties with code.

I faced, however, file loading problems which are rooted in the Rails
convention to match modules/classes to their respective file names.
Hence, I decided to move the Metrics module code into its own file.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Compute average score of a repository

The average score combines all the metric scores so far by averaging
them. For this I have added a list of metrics which need normalization.
When a metric is picked that is part of the list the normalize algorithm
as defined in the Metrics module is applied.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Display average score on the repository report

The average score is now displayed for a chosen repository. This commit
fixes #9.

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