tags: move tags logic to internal pkg and add ParseTags() and Seriali…

…zeTags() helpers (#103)

Move all tags logic to the internal/tags package and add the SerializeTags() and 
ParseTags() helper functions to the mock package. Moving the tags logic was
required to make it accessible by the mock package. 

This commit also increases a bit and adds some examples to the mock package.

Makefile

@@ -29,6 +29,6 @@ quick: # runs all tests without coverage or the race detector
 
 .PHONY: cover
 cover: # runs all tests against the package, generating a coverage report and opening it in the default browser
-	go test -race -covermode=atomic -coverprofile=cover.out ./...
+	go test -covermode=atomic -coverprofile=cover.out -coverpkg github.com/lyft/gostats/... ./...
 	go tool cover -html cover.out -o cover.html
 	which open && open cover.html