Move to a modules/probers model, like the blackbox_exporter. (#34)

There are a number of reasons for this change:
- Modules allow a single instance of the exporter to be configured with numerous
different tls configs. Previously you had to run a different exporter for each
combination.
- Adding new and more complicated options to the exporter should be easier with
modules than if I was to go down the route of accepting configuration directly through url params
- I prefer defining a specific prober (https,tcp) over using the URL to guess
what the user wants

test/https.go

@@ -0,0 +1,42 @@
+package test
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"time"
+)
+
+// SetupHTTPSServer sets up a server for testing with a generated cert and key
+// pair. It returns the server, the cert and key, the path to the ca file and a
+// function to clean up the server.
+func SetupHTTPSServer() (*httptest.Server, []byte, []byte, string, func(), error) {
+	var teardown func()
+
+	testcertPEM, testkeyPEM := GenerateTestCertificate(time.Now().AddDate(0, 0, 1))
+
+	caFile, err := WriteFile("certfile.pem", testcertPEM)
+	if err != nil {
+		return nil, testcertPEM, testkeyPEM, caFile, teardown, err
+	}
+
+	teardown = func() {
+		os.Remove(caFile)
+	}
+
+	// Create server
+	testcert, err := tls.X509KeyPair(testcertPEM, testkeyPEM)
+	if err != nil {
+		return nil, testcertPEM, testkeyPEM, caFile, teardown, err
+	}
+	server := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintln(w, "Hello world")
+	}))
+	server.TLS = &tls.Config{
+		Certificates: []tls.Certificate{testcert},
+	}
+
+	return server, testcertPEM, testkeyPEM, caFile, teardown, nil
+}