Float Sample Rates

[quinnmil]

We should support float values for sample rates to bring it in line with the Ruby agent re: scoutapp/scout_apm_ruby#564

This provides far greater granularity. As is with the Ruby agent, there is a consideration for what to do with an existing rate of 1 - if this value is to be treated as 1% or 100%. The later makes the most sense, so this would require a configuration changes for users wanting to sampling at 1% (1 to .01) and thusly this may warrant a major version bump as this is a "breaking" API change.