Actually use --driver option to set database driver for drone.

This includes refactoring database setup, and migration system.

Remove setupDatabase from main and use `Init` method from database
package.

At database package, defines Init method which actually initiate
database with options given from comand line flag. I think `--path`
wont be used anywhere so I plan to remove it later.

Both meddler and migration initiated here, then we call `Set` method
to setup all the tables, etc. Here I think I want to separate database
schema and turn it into migration script instead, later maybe.

At migration package I made some tweak to `Operation` interface.
Realized that it's ludicrous to let migration driver re-implement `Exec`
and `Query`, I made migration script to receive The whole
migrationDriver struct which contains both Operation implementor, and
the Tx itself. This made possible thanks to Go struct being able to
promote its member, now our migration is more transparent.

There's also stub implementation for bot mysql and postgresql, will
implement this really soon.

cmd/droned/drone.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"database/sql"
 	"flag"
 	"log"
 	"net/http"
@@ -12,13 +11,10 @@ import (
 	"code.google.com/p/go.net/websocket"
 	"github.com/GeertJohan/go.rice"
 	"github.com/bmizerany/pat"
-	_ "github.com/mattn/go-sqlite3"
-	"github.com/russross/meddler"
 
 	"github.com/drone/drone/pkg/build/docker"
 	"github.com/drone/drone/pkg/channel"
 	"github.com/drone/drone/pkg/database"
-	"github.com/drone/drone/pkg/database/migrate"
 	"github.com/drone/drone/pkg/handler"
 	"github.com/drone/drone/pkg/queue"
 )
@@ -66,7 +62,9 @@ func main() {
 	checkTLSFlags()
 
 	// setup database and handlers
-	setupDatabase()
+	if err := database.Init(driver, datasource); err != nil {
+		log.Fatal("Can't initialize database:", err)
+	}
 	setupStatic()
 	setupHandlers()
 
@@ -92,25 +90,6 @@ func checkTLSFlags() {
 
 }
 
-// setup the database connection and register with the
-// global database package.
-func setupDatabase() {
-	// inform meddler and migration we're using sqlite
-	meddler.Default = meddler.SQLite
-	migrate.Driver = migrate.SQLite
-
-	// connect to the SQLite database
-	db, err := sql.Open(driver, datasource)
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	database.Set(db)
-
-	migration := migrate.New(db)
-	migration.All().Migrate()
-}
-
 // setup routes for static assets. These assets may
 // be directly embedded inside the application using
 // the `rice embed` command, else they are served from disk.