Add warning message support for pending remote validations

[aek]

Component: Core
Add warning message support for pending remote validations while keeping the field and form as invalid
Fixes #823 #1044 #1045 #1283 #1344 #1465 #1763 #1806

This tweaks will allow you to show a warning message to the user while the async response to the remote validations get resolved and it will make the valid response to be false if the async response to the remote validation is not ready yet but also it will allow to auto clear the invalid state for the field and the form once the async response is ready by using the remote validation normal flow so if the response is valid the field value is valid and the warning message get cleared automatically by jquery validation or if the response is not valid the error message defined is used instead of the warning message.
Just need to add another rule entry for the custom warning messages or the error default messages will be used instead.
I think that this is the most simply way to solve this problem without having to rewrite the plugin to make it work with deferred/promises. It's a very desirable feature to have deferred/promises support but this will solve the issues and also it could be used for dependency mismatch validations too

[aek]

Please review the errors in travis-ci checks since they are referring to stuffs available in the core itself like comments already available

[staabm]

Thanks for your work.

I dont agree that we should add the concept of warnings which more or less duplicates most stuff our error handling is already capable of.

The only real solution for the "remote pending problem" would be to use a promise based implementation. Thats something we could only add in a new major version because it will not be backwards compatible.

[staabm]

Will close for now because of inactivity. Feel free to ask further questions/provide furhter information regarding this topic here. We can re-open if required.