[lev.DNS] Implement dns module using c-ares, not uv_getaddrinfo

[hnakamur]

What APIs should we have? Are the same APIs as node.js OK?

http://nodejs.org/api/dns.html

[kristate]

Same API as Node.JS is fine, except we will change lookup and add a new option, lookup_family

dns.lookup(domain, callback)
dns.lookup_family(domain, family, callback)
dns.resolve(domain, [rrtype], callback)
dns.resolve4(domain, callback)
dns.resolve6(domain, callback)
dns.resolveMx(domain, callback)
dns.resolveTxt(domain, callback)
dns.resolveSrv(domain, callback)
dns.resolveNs(domain, callback)
dns.resolveCname(domain, callback)
dns.reverse(ip, callback)

[hnakamur]

Tests pass on my Ubuntu 12.04 host.
However, tests fail on my OS X Mountain Lion host.

"ENOTFOUND"
/Users/hnakamur/mysrc/lev/build/lev: tests/test-dns.lua:288: attempt to get length of local 'domains' (a nil value)

"ECONNREFUSED"
/Users/hnakamur/mysrc/lev/build/lev: tests/test-dns.lua:27: attempt to get length of local 'ips' (a nil value)

[hnakamur]

I'm trying to disable tests which fail on Travis.
Do not merge this pull request yet.

[hnakamur]

This time tests passed on Travis (skipping two tests on cname).
Please review and merge pull request #63

[hnakamur]

Thanks for merging!