Add possibilty to "move" instead of "swap" buffers

Thanks Lucas, for putting _buffer-move_ on Github.

I've added a customization flag `buf-move-swap-buffers` which defaults to `t`, which is the current behavior of _buffer-move_. When setting `buf-move-swap-buffers` to `nil`, the buffers are not "swapped", but actually "moved". 

To give an example: when in buffer `#emacs` and doing `buf-move-right`, `#emacs` is ending up in the window where `#gnus` was. And in the window where `#emacs` was, the last buffer is shown. Like this, you can move buffers around (this is especially handy when dealing with many open files or directory trees like _dired_ or _sr-speedbar_).

I don't know it the customization flag is the best solution. Maybe it's even better to provide seperate functions for the two behaviors (something like `buf-move-right` and `buf-swap-right`). Like this, it would be possible to bind these to separate keys and use each behavior alternatively whenever desired. What do you think? I could implement this, maybe you have a good idea for the function names?

BTW: I've also put the "move"-logic to a separate function which is probably nice anyway.