Add a way to use functions with find_by

[HernanRivasAcosta]

In addition to (or perhaps replacing) the syntax discussed on #13 we could have something similar to ets:fun2ms/1 to automatically generate the queries without having to learn a new syntax making it more user friendly. The internal representation of the result of sumo:fun2conds/1 would be arbitrary but it could mimic ets:fun2ms/1.