Change resolve function missing error to warning

[nicolaslopezj]

As talked in #131, resolve function missing validator is now showing a warning instead of an error

[DxCx]

Hi @nicolaslopezj
Please make sure to update the test as well
And make sure that npm test yileds no errors :)

[nicolaslopezj]

I'm not sure on what to do with the tests. Remove them? now the error is no longer required.

Errors:

1) throws an error if a field has arguments but no resolve func
2) throws an error if a field is not scalar, but has no resolve func
3) throws for any missing field if `resolverValidationOptions.requireResolversForAllFields` = true

[stubailo]

Some feedback:

1. Perhaps use console.warn instead of .log?
2. You can test this by overriding console.warn, checking the output, and then overriding it back again.

Thanks for the PR!

[nicolaslopezj]

Now we are ready to go.

Is the message ok? Maybe you are using other pattern

Resolve function missing for "${typeName}.${fieldName}". To disable this 
warning check https://github.com/apollostack/graphql-tools/issues/131

[helfer]

Thanks, this looks fine to me. Could you rebase it so I can merge it?