be_readable, be_writable and be_executable file matchers

You can test a file is readable like this.

```ruby
describe file do
  it { should be_readable }
  it { should be_readable.by('owner') }
  it { should be_readable.by('group') }
  it { should be_readable.by('others') }
end
```

Also you can use be_writable and be_executable matchers as same as be_readable.