Add DNS challenge support for namecheap

[beevik]

This PR adds DNS challenge support for namecheap.

If you decide to accept this PR, please feel free to delay merging it until the changes being made in #144 are complete.

[xenolf]

Thank you for this :)

I hope to be able to land #144 soon(tm) now that I've got a bit more time on my hands.

[xi2]

Hi beevik. I had a quick read through of your code and I wasn't 100% sure it handles TLDs containing a period correctly (such as co.uk or or org.uk). What made me wonder is that given the domain example.co.uk, the splitDomain function would return "co", "uk", nil when I'd imagine it should return "example", "co.uk", nil. These values are then sent through to namecheap.domains.dns.getHosts as SLD and TLD. Am I missing something?

For an alternative way to get TLD and SLD, you can see how I do it in #133 by chopping bits off the front of the FQDN until I hit something that the provider acknowledges, or I believe you can do it by querying nameservers which is what #142 is about?

[beevik]

Ah good catch. I just added code to fix this bug. I decided to use a namecheap API to help determine the TLD/SLD.

[xi2]

Ah cool. That's a nice way of doing it.

We'll see what @xenolf thinks is best for #148, and then we can both change our code accordingly for the timeout issue.

[xenolf]

Hey @beevik! Could you please rebase this PR onto current master and move your provider into lego/providers/dns/namecheap. Thanks!