Create initial residue transaction

https://trello.com/c/WidiT0hp/15--
@shir 
1. residue can't be nil, it is 0 by default, maybe I should call the block if residue > 0?
2. Category type is plain string field in categories table. Do you mean category with a new type, "residue" for example? Category belongs to organization, how can I create it in seeds?