Use readlink to read symbolic instead cd

Introduced readlink to read symbolic link in ~/.marks/ without need to cd to the link location and back. Thanks to a comment of [seidler2547](https://twitter.com/seidler2547/status/497444546926886912).
it adds autocompletion to cd , from [fish docs ](http://fishshell.com/docs/current/)  : 
* `CDPATH`, an array of directories in which to search for the new directory for the `cd` builtin. By default, the fish configuration defines `CDPATH` to be a universal variable with the values . and `~`.