Added short circuit check

Based on the 'debugger' check, this checks for accidental short-circuits often used in debugging.

```
if true && something
if false && something
```

lib/plugins/pre_commit/checks/short_circuit.rb

@@ -0,0 +1,20 @@
+require 'pre-commit/checks/grep'
+
+module PreCommit
+  module Checks
+    class ShortCircuit < Grep
+      def message
+        "Logical short circuit found:"
+      end
+
+      def pattern
+        "^.*([true|false] \&\&)"
+      end
+
+      def self.description
+        "Finds files with a logical short circuit like 'if true &&'"
+      end
+
+    end
+  end
+end

test/files/shortcircuit_file.rb

@@ -0,0 +1,11 @@
+class ShortCircuitFile
+  def blam
+    if true && something
+      puts 'this will always print'
+    end
+
+    if false && something
+      puts 'this will never print'
+    end
+  end
+end

test/unit/plugins/pre_commit/checks/shortcircuit_test.rb

@@ -0,0 +1,22 @@
+require 'minitest_helper'
+require 'plugins/pre_commit/checks/short_circuit'
+
+describe PreCommit::Checks::ShortCircuit do
+  subject{ PreCommit::Checks::ShortCircuit.new(nil, nil, []) }
+
+  it "succeeds if nothing changed" do
+    subject.call([]).must_equal nil
+  end
+
+  it "succeeds if only good changes" do
+    subject.call([fixture_file('valid_file.rb')]).must_equal nil
+  end
+
+  it "fails if file contains true &&" do
+    subject.call([fixture_file('shortcircuit_file.rb')]).to_a.must_equal([
+      "Logical short circuit found:",
+      "test/files/shortcircuit_file.rb:3:    if true && something",
+      "test/files/shortcircuit_file.rb:7:    if false && something"
+    ])
+  end
+end