duplicity-backup.sh: Fix quotation issue

The if condition was not quoted correctly, leading to an error that a unary
operator is expected. This fixes it by quoting the condition correctly, which
is already done in the same way for most of the following if conditions.
Merge pull request #60 from johnpatcher/fix/if-condition-quotation

duplicity-backup.sh: Fix quotation issue
Update README.md - Reduced the wishlist

I think the last point was implemented with #41 ;)
Merge pull request #61 from jaltek/patch-1

Update README.md - Reduced the wishlist
add support for DragonFly and OpenBSD

DragonFly is a FreeBSD derivative, so the same `du` flags as for FreeBSD
and Darwin apply. OpenBSD does not support exclusion.

The correct invocation for in-place `sed` without backup varies. GNU Sed
allows just leaving out the suffix, but BSD Sed requires an empty suffix
(which in turn GNU Sed doesn't accept). OpenBSD Sed has no in-place
option, use `ed` instead.

`mail` works the same on all BSD.

explicitly list `bash` dependency, it's not installed on most BSD by default.