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.


Please revert this, the semicolon is needed here. See [this blogpost](http://blog.izs.me/post/2353458699/an-open-letter-to-javascript-leaders-regarding) for details.