Remove non empty directories

Removing non empty directories becomes necessary
while rewinding certain database operations
such as drop database, remove symbolic link
With patch pg_rewind should work for drop database operation

This is related to 
1: https://github.com/vmware/pg_rewind/issues/33
2: https://github.com/vmware/pg_rewind/issues/34

Also I have already added code for this in the tablespace patch (see remove_directory function in https://github.com/Samrat-Revagade/pg_rewind/commit/1d6b4bb6bbaa1330da57690d45d524e3691c55f9)