Add -e,--sshcmd support to remote commands.

Added support for the rsync-style -e command line option to explicitly
specify the command line bup should use for the remote shell. This
allows the use of advanced SSH options by specifying them on the command
line ala:

	bup init -r user@remotehost -e 'ssh -oCompression=yes'

or the use of alternate shells which accept addressing and command
invocation in the SSH style.

Signed-off-by: Will Rouesnel <w.rouesnel@gmail.com>
Revised documentation to show the new -e,--sshcmd arguments.

Updates the documentation for init, join, save and split to be inline
with the -e,--sshcmd argument and their usage in relation to
-r,--remote.

Also updates the documentation for -r,--remote for init, join, save, split
and on to indicate that user@host notation can be used (no code changes
required it was already possible).

Signed-off-by: Will Rouesnel <w.rouesnel@gmail.com>