Add "skipTldCheck" option for Domain rules.

Allowing validator for local domains like "machine.dev", but the default
behavior of the rule still the same: it validates TLDs, so domains will
only be valid (by default) if the TLD is a valid one.

The signature of rule (using the builder) changes from:

    v::domain() // Still works, same behavior as before.

To:

    v::domain(false) // Skips TLD check.

Fix #33.
Add TLDs "post" and "xxx".
Add support for the ACE prefix on Domain rule.

The "Domain" rule now validates domains on "ASCII Compatible Encoding"
[ACE][1] for internationalized domain names (ICANN, 2003).

An excerpt from Wikipedia article:

> If given a label containing at least one non-ASCII character, ToASCII
> will apply the Nameprep algorithm, which converts the label to
> lowercase and performs other normalization, and will then translate the
> result to ASCII using [Punycode][16] before prepending the four-character
> string ["xn--"][17].

[1]: http://wikipedia.org/wiki/Internationalized_domain_name#ToASCII_and_ToUnicode
[16]: http://tools.ietf.org/html/rfc3492
[17]: http://www.atm.tut.fi/list-archive/ietf-announce/msg13572.html