Add support for new locales - Update monthly.js

[jefferson]

Hi! Sorry about make commit in this way, but I didn't see any instruction about how make contributions here.
Now we have support for locales :)

We can use:
$('#mycalendar2').monthly({monthNames: 'pt-br'})

Default is 'en'.

[kthornbloom]

Thank you! I will check this out & merge it in soon

[richardtallent]

We should implement this using Intl.DateTimeFormat for both the short month name and short weekday names, with fallback to English for the few modern browsers (Safari desktop) that don't yet support that standard. That way we don't have to carry all of these localized arrays in the code. Similarly, options.weekStart should be a number, not an English abbreviation.

Edit: I made the changes described above in pull request #41.