Add 'fasd' (remove 'z')

Command-line productivity boosted once again with 'fasd', replacing 'z'.
Installs fasd locally (as a submodule), and prezto's module supports it.

Commands summarized:

- f : list recent file (`f` is also useful)
- s : list or select recent file or directory interactively (`s` is also useful)
- z : change the current working directory
- zz/j : change the current working directory interactively

Completion with tab:

- ,{pattern}  : auto-completes anything matching {pattern}
- f,{pattern} : auto-completes files matching {pattern}
- d,{pattern} : auto-completes directories matching {pattern}