Rename cmd agent and server folders and binaries (#330)

Renamed `cmd/drone-agent` to `cmd/agent` and `cmd/drone-server` to `cmd/server` and binaries to get rid of the drone name.

cmd/server/main.go

@@ -0,0 +1,42 @@
+// Copyright 2018 Drone.IO Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/woodpecker-ci/woodpecker/version"
+
+	"github.com/joho/godotenv"
+	_ "github.com/joho/godotenv/autoload"
+	"github.com/urfave/cli"
+)
+
+func main() {
+	godotenv.Load(".env")
+	app := cli.NewApp()
+	app.Name = "woodpecker-server"
+	app.Version = version.String()
+	app.Usage = "woodpecker server"
+	app.Action = server
+	app.Flags = flags
+	app.Before = before
+
+	if err := app.Run(os.Args); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}