Refactor parts of the code and add more tests

This is a refactor of parts of the code, some minor things such as adding
punctuation in the end of sentences and making the code a bit easier to read.

More tests has been added in the form of gometalinter which checks the code
health, style and spelling errors. The flaky cleanup test should also be fixed
with this commit.

cleanup.go

@@ -38,11 +38,11 @@ func (db *PGStore) cleanup(interval time.Duration, quit <-chan struct{}, done ch
 	for {
 		select {
 		case <-quit:
-			// Handle the quit signal
+			// Handle the quit signal.
 			done <- struct{}{}
 			return
 		case <-ticker.C:
-			// Delete expired sessions on each tick
+			// Delete expired sessions on each tick.
 			err := db.deleteExpired()
 			if err != nil {
 				log.Printf("pgstore: unable to delete expired sessions: %v", err)