Ensure we are dealing float datapoints

Convert strings to floats, but in a reliable manner.

Float() will only convert things that look like floats, whereas to_f is
a bit more leniant (eg nil.to_f == 0.0; Float(nil) throws an exception).
Add --method option, default to 'median'

Allow specification of a 'check method', so we can pick the right way
to discover the value to check against our thresholds. These vary
depending on the check needed.

* max
* min
* median (set as the default)
* mean
* last
* penultimate (throw away the last value, graphite sometimes corrupts
  it)