web-cmd.py: encode path to utf-8 before path traversing

If there are Non-ASCII characters in path, fail to traverse to it, and
will raise a "UnicodeEncodeError" error when:

        print 'Handling request for %s' % path

Encode path to "utf-8" will fix this.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
User can specify remote shell by BUP_SSH environment

When running bup on remote server, user may like to specify a remote
shell other than the default "ssh" to be executed.  Add a new
environment variable BUP_SSH to hold the custom ssh command, so that
the user can use her/his prefer ssh command.  E.g.

If user doesn't like the default ssh behavior, such as strict host-key
checking, she/he can prepare a ssh config file as follows:

        CheckHostIP                 no
        StrictHostKeyChecking       no

, and defines BUP_SSH environment like:

        $ BUP_SSH="ssh -F /path/to/custom/ssh_config"
        $ export BUP_SSH

Then when running a remote bup, it will never fail because of unknown
ssh host-key or changed ssh host-key.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>