[qtcontacts-sqlite] Check for read/write capability using QFileInfo

Previously, QDir::isReadable() was used.  We should ensure that we
have both read and write capability, using QFileInfo.  This is also
more accurate, as it will work if we have Group permissions.
Merge pull request #89 from chriadam/gread

[qtcontacts-sqlite] Check for read/write capability using QFileInfo