Support Sample Values < 1

[quinnmil]

Users have requested we support sampling values < 1. Currently, the sampling configuration supports only integer values 1-100.

A proposed plan for updating this would be that we:

1. Convert any setting >= 1 to a float rate (i.e. divide by 100)
2. Always compare against rand(0,1).

Additionally, documentation should be updated to suggest that people use a rate between 0 and 1 for new values, e.g. 0.5 = 50%