feature: the statistic should respect the user mapping of the repository

Redmine already can map users to committers (applied to each changeset).
The statistic now respects this mapping by merging the data for all
committers which alias the same user.
removed the 1 year window for the commits_per_month statistic

the other graphs (commits_per_hours, commits_per_weekday, ...) don't have
such an window too
Merge branch 'feature-improved-statistic' of https://github.com/niosHD/redmine_git_hosting into niosHD-feature-improved-statistic
Merge branch 'devel' into niosHD-feature-improved-statistic