[os] path.isSymLink()

also more unit tests

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>
[os] lastModified (mtime) functions

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>
[os] cptree accept options

now it accepts option 'preserve' and 'deref'

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>
os unit tests

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jollamobile.com>