Provide "rounded" durations in report view

[Bytekeeper]

Round durations to an configurable interval (default 5mins). Use "round half to even" to break ties: http://en.wikipedia.org/wiki/Rounding#Tie-breaking

• This might lead to the total duration being smaller or bigger than the real duration. Show this below (optionally: Show difference in red if it's less and blue if it's more than the real total duration)