New feature: Holiday importer / API module

[shridhar-tl]

Need a feature to import list of holidays for individual timezone . Also need to integrate with 3rd party API's to automatically identify holidays.

Additionally need option to import/enter leaves for team members which will be useful in reports.

This feature will be implemented based on popularity. Anyone interested in this feature kindly +1 this issue. Also any suggestions related to UI implementation / API are welcome.