Dispense with reverse IP addresses

For PTR records, we need to reverse IP addresses. I want to dispense with reversed IP addresses.

I want to pass both examples.
```
$ bundle exec rspec

74.125.28.27
  should have the correct dns entries with {:type=>"PTR", :domainname=>"pc-in-f27.1e100.net", :with=>"exceptions"}

27.28.125.74.in-addr.arpa.
  should have the correct dns entries with {:type=>"PTR", :domainname=>"pc-in-f27.1e100.net"}
```

I will do the followings.
- [x] Add an example to README.md
- [x] ~~Add spec for not reversed IP address case~~ (This is too detail)
- [x] Add implementation

------
## Investigation
For example, `gmail-smtp-in.l.google.com` is `74.125.28.27`, `74.125.28.27` can provide `pc-in-f27.1e100.net` domainname in PTR record.

### dig
Dig can resolve without reversed IP address.
```
$ dig -x 74.125.28.27 +short
pc-in-f27.1e100.net.
$
```

###  dnsruby
Dnsruby can resolve without reversed IP address when type is PTR.
```
require 'dnsruby'
resolver =  Dnsruby::Resolver.new
begin
  resolver.query('74.125.28.27', Dnsruby::Types.ANY)
rescue => e
  e
end
=> #<Dnsruby::NXDomain: Dnsruby::NXDomain>

resolver.query('27.28.125.74.in-addr.arpa.', Dnsruby::Types.ANY).answer[0].instance_variable_get('@domainname')
=> #<Dnsruby::Name: pc-in-f27.1e100.net.>

resolver.query('74.125.28.27', Dnsruby::Types.PTR).answer[0].instance_variable_get('@domainname')
=> #<Dnsruby::Name: pc-in-f27.1e100.net.>
```

### rspec-dns
Rspec-dns can resolve only with fully reversed IP address.
```
rspec-dns
require 'rspec'
require 'rspec-dns'
describe '74.125.28.27' do
  it { is_expected.to have_dns.with_type('PTR').and_domainname('pc-in-f27.1e100.net') }
end

describe '27.28.125.74.in-addr.arpa.' do
  it { is_expected.to have_dns.with_type('PTR').and_domainname('pc-in-f27.1e100.net') }
end
```

```
$ bundle exec rspec

74.125.28.27
  should have the correct dns entries with {:type=>"PTR", :domainname=>"pc-in-f27.1e100.net", :with=>"exceptions"} (FAILED - 1)

27.28.125.74.in-addr.arpa.
  should have the correct dns entries with {:type=>"PTR", :domainname=>"pc-in-f27.1e100.net"}
```