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
Error bar support for the histogram plot.

In the histogram syle configuration there will be an option to overlay
error bars on top of the existing plot.  This uses the flot errorbars
plugin. The error bars are disbled when derivatives are enabled.

The errors are computed from the min and max values returned by ES and
comparing it against whatever chart value is currently active. Not sure
that it makes much sense to plot errors against anything other than mean.
Add zoom out to timepicker panel at top of page.

This can help keep sub-panels uncluttered.