Method to delete local cache

[jayqi]

If you're processing a lot of data in a loop and want to reclaim disk space after you're done with one path, then it could be helpful to have a way to delete the locally cached file. I don't think this should cause any problems because other methods that use the cache will check if it exists and download.

Currently, one can do this with cloud_path._local.unlink() but it would be better to have a method in the public API for this.

[pjbull]

Consolidated to #10