remove artificial restriction of path length

let the filesystem handle this instead
Fix issue #12: Sender throws IllegalArgumentException if source dir has only 1 component

Fixes https://github.com/perlundq/yajsync/issues/12

The bug was caused by wrongly assuming the namecount of / would be 1,
but in reality it is 0.

Additional changes: Always check at runtime that sub is a proper
subdirectory of parent. Remove restriction that parent must be an
absolute path.