Errors

Please take a look on my implementation of errors feature in js-schema library.
This pull request should be treated as starter for discussion with you about errors API implementation.

I'm aware my implementation is far from perfect and not complete, but let's try to polish it together.


In the beginning I was thinking what kind of structure I should return when error appears.
First I returned  
```
{ 
  message:message,
  schema:schema,
  input:input
}
```

but then realized that simple message is good enough.
This function is relic from my trials.
In that case it is not necessary and I can remove it.