Various improvemes:

* Update Readme.
* Add option to reset window configuration when switching to perspective
  without window configuration(for example newly created
  perspective). See `persp-reset-windows-on-nil-window-conf'.
* Customizable function to get list of buffers: `persp-buffer-list-function'.
* Add `with-persp-buffer-list' macro, `persp-buffer-list-function'.
* Other small changes.