- Clarify and correct direction of trading:
- Do not exchange "costbuy" and "costsell".
--  Shop buys  : player sells : at costbuy  : with buttonsell
--  Shop sells : player buys  : at costsell : with buttonbuy
- Fix barter shop text display.

Signed-off-by: Xiong Changnian <xiong@cpan.org>
Merge pull request #6 from Xiong/pull-clarity

- Clarify and correct direction of trading. See https://github.com/Kotolegokot/minetest-mod-money/issues/5