Added any.label method

[agraddy]

Added any.label() method as described in #414.

It looked like language.root value was already available in the error message, but it was just not taking precedence over the original key. I changed the order from this:

var localized = this.options.language;
this.context.key = this.context.key || localized.root || Language.errors.root;

To this:

var localized = this.options.language;
this.context.key = localized.root || this.context.key || Language.errors.root;

I then added the any.label() method as a convenience method. I believe it should be set up so that language.root takes precedence over any.label().

[phated]

Just ran up against this. Would like to see something like this land.

[Marsup]

This is merged. I reworked the readme a bit because Joi.firstName() doesn't exist yet ;)
Sorry for the delay, I'm still catching up on all the issues.

[lock]

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