Break timeHelpers into smaller functions

Code from @nickstenning, introduce the breakpoints array and search
in it for the needed fuzzystring and when to update next instead of
a complex else-if tree.

timeHelpers service has two functions:
- toFuzzyString: returns the fuzzy time string
- nextFuzzyUpdate: returns the next time to update (in sec)
Remove dateFilter

Since we use the fuzzyString for the timestamps and
momentjs for the tooltip the date filter was actually
never used.