Implement Portable Mode

According to these specifications:
Portable mode only applies to Windows (for now)
Portable mode only applies if ATOM_HOME is not set
If there is a .atom directory as sibling to directory with running process
	use that as ATOM_HOME

spec/atom-portable-spec.coffee

@@ -0,0 +1,52 @@
+path = require "path"
+fs = require 'fs-plus'
+temp = require "temp"
+rimraf = require "rimraf"
+AtomPortable = require "../src/browser/atom-portable"
+
+fdescribe "Portable Mode", ->
+  describe "Windows", ->
+    platform = "win32"
+
+    describe "with ATOM_HOME environment variable", ->
+      environmentAtomHome = "C:\\some\\path"
+      it "returns false", ->
+        expect(AtomPortable.isPortableInstall(platform, environmentAtomHome)).toBe false
+
+    describe "without ATOM_HOME environment variable", ->
+      environmentAtomHome = undefined
+      portableAtomHomePath = path.join(path.dirname(process.execPath), "../.atom").toString()
+      portableAtomHomeNaturallyExists = fs.existsSync(portableAtomHomePath)
+      portableAtomHomeBackupPath = portableAtomHomePath + ".temp"
+
+      beforeEach ->
+        fs.renameSync(portableAtomHomePath, portableAtomHomeBackupPath) if fs.existsSync(portableAtomHomePath)
+
+      afterEach ->
+        if portableAtomHomeNaturallyExists
+          fs.renameSync(portableAtomHomeBackupPath, portableAtomHomePath) if not fs.existsSync(portableAtomHomePath)
+        else
+          rimraf.sync(portableAtomHomePath) if fs.existsSync(portableAtomHomePath)
+        rimraf.sync(portableAtomHomeBackupPath) if fs.existsSync(portableAtomHomeBackupPath)
+
+      describe "with .atom directory sibling to exec", ->
+        beforeEach ->
+          fs.mkdirSync(portableAtomHomePath) if not fs.existsSync(portableAtomHomePath)
+        it "returns true", ->
+          expect(AtomPortable.isPortableInstall(platform, environmentAtomHome)).toBe true
+
+      describe "without .atom directory sibling to exec", ->
+        beforeEach ->
+          rimraf.sync(portableAtomHomePath) if fs.existsSync(portableAtomHomePath)
+        it "returns false", ->
+          expect(AtomPortable.isPortableInstall(platform, environmentAtomHome)).toBe false
+
+  describe "Mac", ->
+    platform = "darwin"
+    it "returns false", ->
+      expect(AtomPortable.isPortableInstall(platform, platform)).toBe false
+
+  describe "Linux", ->
+    platform = "linux"
+    it "returns false", ->
+      expect(AtomPortable.isPortableInstall(platform, platform)).toBe false