Normalize option to diversity metrics from 0 to 1

[dvro]

Diversity class (https://github.com/viisar/brew/blob/master/brew/metrics/diversity/base.py) accepts several diversity metrics, but different diversity metrics have different behavior (some is from 0 to 1, being 1 the most diverse; others 1 to -1, being -1 the most diverse ...).

In this feature, we want the "normalized" option, in which all methods would return a float in the range 0 to 1, being 1 the most diverse. This will be useful for generation methods using Diversity metrics (example).