improve quality of the sampler

* rename Site to Target for clarity
* godoc for sampler
* tests for sampler
Merge pull request #10 from canaryio/improve-sampler

improve quality of the sampler
readme: godoc badge
Merge pull request #11 from canaryio/godoc-badge

readme: godoc badge
add MIT license
Merge pull request #12 from canaryio/license

add MIT license
rename canary.go -> main.go
introduce a minimal canaryd
restructure the Sensor for better composition
enable sensors to be stopped
start getting READMEs in line