Fix handling of correctly encoded unicode filenames (#534)

Fix handling of correctly encoded unicode filenames (#534)

Tahoe CLI commands working on local files, for instance 'tahoe cp' or 'tahoe
backup', have been improved to correctly handle filenames containing non-ASCII
characters.

In the case where Tahoe encounters a filename which cannot be decoded using the
system encoding, an error will be returned and the operation will fail.  Under
Linux, this typically happens when the filesystem contains filenames encoded
with another encoding, for instance latin1, than the system locale, for
instance UTF-8.  In such case, you'll need to fix your system with tools such
as 'convmv' before using Tahoe CLI.

All CLI commands have been improved to support non-ASCII parameters such as
filenames and aliases on all supported Operating Systems except Windows as of
now.