In the Knapsack voting style, we can't add an investment if its cost is
greater than the money we've got left, but in other voting styles money
might not be the issue.
So we're introducing the term "resources" and adapting the code
accordingly.
Since we're going to introduce a new voting style which will not be
based on money, we're extracting the logic specific to the current
voting style to a new class.
This way adding new voting styles will be easier.
