Add Powershell as a supported shell

- There may be additional functionality to implement, but I've been
  running this way for a month with no known issues

tests/shells/test_powershell.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+
+import pytest
+from thefuck.shells import Powershell
+
+
+@pytest.mark.usefixtures('isfile', 'no_memoize', 'no_cache')
+class TestPowershell(object):
+    @pytest.fixture
+    def shell(self):
+        return Powershell()
+
+    def test_and_(self, shell):
+        assert shell.and_('ls', 'cd') == '(ls) -and (cd)'
+
+    def test_app_alias(self, shell):
+        assert 'function fuck' in shell.app_alias('fuck')
+        assert 'function FUCK' in shell.app_alias('FUCK')
+        assert 'thefuck' in shell.app_alias('fuck')