Add pre/post operation hooks

[henris42]

For security purposes, we'd like to have a possibility to do authorization of operations on WebDAV filesystem call (CRUD) level. Also for logging purposes, we'd like to have same possibility post-operation.

We are proposing, that a pro/post hook interface to be added to the library, so we have a flexible way to do use cases like this. Hooks should be available for all WebDAV supported operations.

[messense]

Does #33 work for you?