- Major change to file_mru

1. Time to save:
- Before: Save to file on every event
- Now: Use VimLeavePre to save.

2. maintain two list, short one for quick start; long one for keep history.
use `file_mru:*` or `all`, `long`, `_`  to load.

3.
Modularize and Merge file_mru / directory_mru into one
Rename file_mru directory_mru to mru/file and mru/directory
(mru) Set update interval
Update doc for mru code overhaul
Merge branch 'master' of https://github.com/Shougo/unite.vim into feature/lazy_mru
doc fix
Merge pull request #200 from zhaocai/feature/lazy_mru

Feature: lazy mru