Add experimental Riex.Set

cc/ @ammmir

What do you think about this API (on the `set_test.exs` file) ?

I thought about using `Riex.put` for every saving on Riak and find to fetch, it doesn't matter if it's for a `Riex.Object` or a `Riex.Set` (and soon the other CRDTs). I think it's good the have a consistent API.

(For now tests for CRDTs will be excluded pass on Travis CI as they do not have Riak 2.0.*)

To play with this test you need to:

```
riak-admin bucket-type create set_bucket '{"props":{"datatype":"set"}}'
riak-admin bucket-type activate set_bucket
```
 
Any thoughts?