remote validations

Add support to use remote validations when Rails application has been mounted on a custom path. 

For example one of my applications has the config.ru like this
```
map '/useapp/accounting' do
        require ::File.expand_path('../config/environment',  __FILE__)
        run Accounting::Application
end
```
In this case the standard remote validations does not work because the ajax call must be sent to '/useapp/accounting/validators/uniqueness' and not to '/validators/uniqueness'.

All tests are successfully passed. This change does not need other tests.