Add duration literal support

[fmarcia]

This pull request adds support for duration literal:

-1h == -1 * 1h
+1h == 1h
2 * 1h == 2h
1h + 1m == 1h1m
1h - 1m == 59m
1h / 10m == 6
1h / 2 == 30m

round, max, min and abs builtin functions have been updated to support duration as well.

[antonmedv]

I'm not sure yet if this should be added directly to the language itself. We already have duration() func.

Why do you think this should be implemented directly in the language?

[fmarcia]

Mostly for the sake of readability. I'm using durations frequently, and I find literal format more readable and quicker to type as well.