mantash should support `~~/` syntax

Specifying `mantash ls ~~/` throws a Permission denied error.

```
$ mantash -v ls ~~/
manta.client: DEBUG: ListDirectory '/joybdha/stor'
...
  File "/opt/pkg/lib/python2.7/ssl.py", line 542, in __init__
    self._context.load_verify_locations(ca_certs)
IOError: [Errno 13] Permission denied
```
