New features for graphite check

Changed functionality so that each check (with exception to increasing) may be configured with a -g option to specify that the value should not be greater than whatever the check is calculating. This functionality defaults to less than.

Added a percentile check whereby the percentile to be calculated may be specified along with the a percentage by which the last value may be greater or less than the percentile.

Added a percentage above average check whereby a percentage of the last value may not be greater or less than the mean over the time series.