Aligned cursors implementation

[toptobes]

Tests haven't been updated, nor have Table/Collection been fully updated. I can't even guarantee the new cursors are functional in their current state but it serves as an idea of what C# cursors could look like if they were ported from the other clients, while still keeping LINQ in mind (so functionality like mapping and converting to a list is deferred to LINQ)

Actually (as of April 23) a rather complete suite of tests is added (essentially ported from other clients).

Fixes #69 (if the general approach is approved)

[sl-at-ibm]

@toptobes in cursor-world, there is no "silent filter overwrite" (as opposed to what I found for FindOne, cf. #156 ). The only similar pattern is the following:

var cursor = collection.Find(filter_ONE).Filter(filter_TWO)

In this case, filter_ONE is lost and entirely replaced by filter_TWO. I think we can live with that (incidentally, this is what other clients do as well, to my understanding). Supplying a further .Filter(...) is a very intentional action by the user.
Would you agree? If you do, then this comment is resolved :)

[sl-at-ibm]

LGTM!