Move to Nexendrie

Move the project under @nexendrie organization.

• Move the repository.
• Translate repository's description to English.
• Create new package on Packagist, deprecate the old one.
• Add new component on componette, deprecate the old one, planette/componette#189.
• Change namespace to Nexendrie\EventCalendar.
• Add GitLab pages with documentation (#1).
• Use issue tracker and merge requests on GitLab.
• Update Travis's configuration.
• Setup Scrutinizer.
• Add contributing guide.