App version management.

* DB migration adds app_versions table for managing known app versions.
* /ping reads app version from User-Agent header and responds with latest version if the app is OKO and its version is older than the latest one stored.
* Added endpoint /app-versions which
  * lists all known versions via GET
  * adds/updates a version via POST

#3
Fix #4

models/models.go

@@ -4,6 +4,7 @@ import (
 	"io"
 	"time"
 
+	"github.com/coreos/go-semver/semver"
 	geojson "github.com/paulmach/go.geojson"
 )
 
@@ -40,6 +41,11 @@ type BuildInfo struct {
 
 // transport objects
 
+type AppVersionInfo struct {
+	Version semver.Version `json:"version"`
+	Address string         `json:"address"`
+}
+
 type Coords struct {
 	Lat float64 `json:"lat"`
 	Lng float64 `json:"lng"`