Add zap logger, migrate some constants up a level

const.go

@@ -0,0 +1,29 @@
+package main
+
+import "os"
+
+const (
+    Version = "0.0.1"
+
+    // DefaultFilePerms are the file perms used for created files.
+    DefaultFilePerms os.FileMode = 0644
+
+    // DefaultFolderPerms are the file perms used for created folders.
+    DefaultFolderPerms os.FileMode = 0755
+
+    // ServersPath is the path of the servers within the configured DataPath.
+    ServersPath = "servers"
+
+    // ServerConfigFile is the filename of the server config file.
+    ServerConfigFile = "server.json"
+
+    // ServerDataPath is the path of the data of a single server.
+    ServerDataPath = "data"
+
+    // DockerContainerPrefix is the prefix used for naming Docker containers.
+    // It's also used to prefix the hostnames of the docker containers.
+    DockerContainerPrefix = "ptdl-"
+
+    // WSMaxMessages is the maximum number of messages that are sent in one transfer.
+    WSMaxMessages = 10
+)