Added support for selling items at the merchant

[itzandroidtab]

Added support to allow the user to sell items. (selecting item to sell, pressing sell button). This does not add support for the call that processes the sold item. I wanted to split this in a different PR because the other part is dependent on having a inventory.

@Snaacky One of the files is not perfectly formatted (merchant.py). Otherwise the file would be 900 lines long for no reason.

This PR should be merged after #70 as that fixes a bug with large packets