ReverseIP: add reverse lookup for bare IP4 addresses.

There are some potential problems with this:

- DNS lookup can be slow, although we try to mitigate this by only
  waiting for up to 1 second for a response.
- It's more or less uncacheable in the application, given the wide
  search space and unknown cache invalidation parameters.
- Results will depend on the resolver set up.  This may include leaking
  some information about specialized internal DDG reverse lookup, if any.
- Returns FQDN, which is both correct and possibly not exactly what the
  searcher might expect.

This version only handles well-formed IP4 addresses.  It could be easily
expanded to include IP6, but I wanted to make sure this was what was
being sought before adding more matching.

------
**What does your instant answer do?**

Returns the reverse DNS information when a user queries for a bare IP address.

**What problem does your instant answer solve (Why is it better than organic links)?**

Less clicking through to potentially questionable sites which provide the same service.

**What is the data source for your instant answer? (Provide a link if possible)**

The DNS.  

**Why did you choose this data source?**

It's pretty much authoritative for this kind of info.

**Are there any other alternative (better) data sources?**

Possibly, if the actual request is for WHOIS info on the discovered address.

**What are some example queries that trigger this instant answer?**

127.0.0.1
46.51.216.186

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**

People who read logs (mostly sysadmins) and the overly curious

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**

It may address [738](https://duck.co/ideas/idea/734/reverse-dns-an-ip-address).  Considering that an API is thought to be required, I am not sure that it does.

**Which existing instant answers will this one supercede/overlap with?**

Currently, Wolfram|Alpha returns an odd answer like: ```Date: 127.0.0.1 (IPv4 address), assuming 127.0.0.1 is referring to an internet site.```

**Are you having any problems? Do you need our help with anything?**

I'm not sure that this is actually a goodie, since it ostensibly requires network access. Even though it is lighter weight access than something like HTTP, the supplied timeout might lead to uncertainty as to whether or not we'll be able to show the answer.

![revip](https://f.cloud.github.com/assets/6280000/1948977/a29f0488-80d0-11e3-9684-300861a311ed.png)

```
[✔] Added metadata and attribution information
[✔] Wrote test file and added to t/ directory
[✔] Verified that instant answer adheres to design guidelines(https://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[] Tested cross-browser compatability
```