Add checksum to Player.

Closes #478.

checksum.js

@@ -0,0 +1,18 @@
+var crypto = require('crypto')
+
+module.exports = function (checksum) {
+    if (typeof checksum == 'function') return checksum
+    var algorithm
+    switch (algorithm = checksum || 'sha1') {
+    case 'none':
+        // todo: return null
+        return function () {
+            return {
+                update: function () {},
+                digest: function () { return '0' }
+            }
+        }
+    default:
+        return function (m) { return crypto.createHash(algorithm) }
+    }
+}

player.js

@@ -2,9 +2,11 @@ var ok = require('assert').ok
 var fs = require('fs')
 var path = require('path')
 var cadence = require('cadence/redux')
+var checksum = require('./checksum')
 
-function Player (directory) {
-    this.directory = directory
+function Player (options) {
+    this.directory = options.directory
+    this.checksum = checksum(options.checksum)
 }
 
 // todo: outgoing
@@ -60,7 +62,7 @@ Player.prototype.readEntry = function (sheaf, buffer, isKey) {
     }
     ok(!count, 'corrupt line: could not find end of line header')
     var fields = buffer.toString('utf8', 0, i - 1).split(' ')
-    var hash = sheaf.checksum(), body, length
+    var hash = this.checksum(), body, length
     hash.update(fields[2])
     if (buffer[i - 1] == 0x20) {
         body = buffer.slice(i, buffer.length - 1)

strata.js

@@ -24,7 +24,7 @@ function extend(to, from) {
 }
 
 function Strata (options) {
-    options.player = new Player(options.directory)
+    options.player = new Player(options)
     options.logger = new Logger(options.directory, this.sheaf)
     options.logger._sheaf = this.sheaf = new Sheaf(options)
 }