Fixes histogram plot disappears when time range is too small (issue #733)

This fixes rounding behaviour in interval_to_ms. The interval_to_ms
function will round any fractional component of an interval to 0 because
it tries to parse it as an integer.

I've fixed it by trying by parsing as a float but then trying to preserve
the existing behaviour of returning integer results in the callers.

closes elasticsearch/kibana/#733