Simplify Funder interface

[matthiasgeihs]

It is currently rather complicated to setup a funder.

The constructor NewFunder takes as input accounts map[Asset]accounts.Account and depositors map[Asset]Depositor, both mapping from the same list of assets to a set of objects. For ease of use, these two maps could be combined into one map by integrating the account into the depositor.

Furthermore, there are no constructors for these maps and not much documentation on NewFunder, so it is hard to know how exactly the maps should be created. For ease of use, we should extend the documentation of NewFunder and we should make it easier to create a depositor map, e.g., by adding a special type for it with respective helper methods.