Added localization for datepicker

DatePicker plugin features an option for including text to display on the trigger button: "buttonText".
Since it would be nice if it were possible to have this message to change when varying users' locale, I added translation filter to the configuration passed from config.yml file. 

Usage:

# app/config/config.yml
    date: 
        enabled: true
        configs:
            buttonText: mydate_jquery_text

# message.en.yml
    mydate_jquery_text: "Select a Date"

Hope this will be useful.