Adds symbolic link resolution on backup sources

Because Duplicity doesn't resolves symbolic link when used as source, one had to
use real paths (id. the target of the symbolic link) in config file, which can
be somehow painful.
Given sources are now tested for being a symbolic link, if so, symbolic link
target path is resolved and used as backup source instead of symbolic link, note
that in thise case, "--allow-source-mismatch" Duplicity option will be
automatically used (otherwise Duplicity would abort this backup).

Fixes #2
Merge branch 'feat/2-duplicityBackup_follow_source_symlinks'