Add a few functions thats revolving around Either to accumulate errors

[afsalthaj]

1. def orElse(either1, either2) (try one either, or else ty the next one)

2. validation data type ==> Left of Either lifted to a List

3. Applicative for validation to accumulate errors.

Please excuse me if some of these are already available in some other way.

@dbrattli I am happy to contribute if you are ok with it.

[dbrattli]

Sounds great!