Alias validator

[offirgolan]

Creates an alias between a single attribute's validations to another.

 // Examples
 validator('alias', 'attribute')
 validator('alias', {
   alias: 'attribute'
 })

@blimmer Let me know what you think. I went kinda crazy and cleaned up some stuff.

The important files are:

• addon/validations/factory.js
• addon/validators/alias.js
• tests/integration/validations/model-relationships-test.js

Resolves #160

[blimmer]

nice - i think this is the right behavior vs. firstObject.

[blimmer]

This LGTM cc @gustaff-weldon

[offirgolan]

I feel like there will be a use-case where you might not want to
include every error message in the aliased validation. What do you think of
adding a firstMessageOnly option or something like that?

[blimmer]

I think the option is reasonable, or just documentation around firstObject. It's a pretty common ember pattern to use to get the first object from an array.

[offirgolan]

@blimmer not sure I follow what you mean.

[blimmer]

Sorry, just reread my comment and it wasn't very clear. I was thinking that a firstMessageOnly will really just get the messages and then call .firstObject on the array. I think most ember folks know about it, but I think the option is a good idea for validations that you always want only one message. I could see people using that as a global validation option, too, so that plan sounds good to me!