Support different thresholds for the same quality model

[acs]

A clear use case is a user that want to reuse an already existing quality model but she wants to change the thresholds.

[acs]

Probably the best design is:

• The metric has the default thresholds
• If the user defines different thresholds, they are stored in a new object: user, metric, thresholds
• Once a user assess a project, if she has custom thresholds, use them

[acs]

Depends on #33