Fix issue when sorting by time with '--tree' option

When sorting by 'cpu_times', stats are returned as a pcputimes namedtuple,
with user and system CPU time, so they must be summed before numerical
comparison.

Close #453.
Merge branch 'desbma-develop' into develop
glances_fs.py: simplify for loop iteration
Merge branch 'develop' of github.com:nicolargo/glances into develop