[PoC] Add traditional calendars support and related twig functions

Q  | A
------------- | -------------
Bug fix?  | no
New feature?  | yes
BC breaks? | no
Deprecations? | no
Fixed tickets | -
License | MIT
Doc PR | -

This is just a proposal to add support for traditional calendar systems into Sylius. I'm so new to Symfony2 so please forgive me if you see some bad practices ;)

I've also added some useful twig filters such as `sylius_rtl` which checks if current locale is RTL so that we can load RTL stylesheets and whatnot.

Your comments are really appreciated ;)

Here is a list of twig filters/functions I've proposed:
* `date` Override's Twig default date filter with calendar-aware conversion. (Based on current locale. I'm not sure if this is a good idea!!)
* `sylius_language()` Returns localized language name of current locale code.
* `sylius_rtl()` Returns true if current locale code is of an RTL-type language.
* `sylius_direction()` Returns current locale language direction. (`rtl`, `ltr`)
* `sylius_calendar()` Returns current locale calendar system. (`gregorian`, `persian`, `hebrew`, etc)

* I've also overridden symfony's `date` form type to support calendar-aware dates.