Add basic Linux support

Copies both the electron-prebuilt distribution into the app's toplevel
dist directory (by default) and the user's app into the correct subdirectory.

index.js

@@ -22,6 +22,7 @@ module.exports = function packager (opts, cb) {
 
   switch (os.platform()) {
     case 'darwin': platform = require('./mac'); break
+    case 'linux': platform = require('./linux'); break
     default: cb(new Error('Unsupported platform'))
   }
 

linux.js

@@ -0,0 +1,71 @@
+var path = require('path')
+var fs = require('fs')
+var child = require('child_process')
+var mkdirp = require('mkdirp')
+var ncp = require('ncp').ncp
+
+module.exports = {
+  createApp: function createApp (opts, cb, electronPath) {
+    var templateApp = path.join(electronPath, 'dist')
+    var finalDir = opts.out || path.join(process.cwd(), 'dist')
+    var userAppDir = path.join(finalDir, 'resources', 'default_app')
+    var originalBinary = path.join(finalDir, 'electron')
+    var finalBinary = path.join(finalDir, opts.name)
+
+    function copyApp () {
+      mkdirp(finalDir, function AppFolderCreated (err) {
+        if (err) return cb(err)
+        copyAppTemplate()
+      })
+    }
+
+    function copyAppTemplate () {
+      ncp(templateApp, finalDir, {filter: appFilter}, function AppCreated (err) {
+        if (err) return cb(err)
+        copyUserApp()
+      })
+    }
+
+    function copyUserApp () {
+      ncp(opts.dir, userAppDir, {filter: userFilter}, function copied (err) {
+        if (err) return cb(err)
+        if (opts.prune) {
+          prune(function pruned (err) {
+            if (err) return cb(err)
+            renameElectronBinary()
+          })
+        } else {
+          renameElectronBinary()
+        }
+      })
+    }
+
+    function renameElectronBinary () {
+      fs.rename(originalBinary, finalBinary, function electronRenamed (err) {
+        if (err) return cb(err)
+      })
+    }
+
+    function prune (cb) {
+      child.exec('npm prune --production', { cwd: userAppDir }, cb)
+    }
+
+    function appFilter (file) {
+      return file.match(/default_app/) === null
+    }
+
+    function userFilter (file) {
+      var ignore = opts.ignore || []
+      if (!Array.isArray(ignore)) ignore = [ignore]
+      ignore = ignore.concat([finalDir])
+      for (var i = 0; i < ignore.length; i++) {
+        if (file.match(ignore[i])) {
+          return false
+        }
+      }
+      return true
+    }
+
+    copyApp()
+  }
+}