Completed MergeVars implementation

* I added methods to Update Merge Var, Reset and Set the merge var.
* I removed `MergeVarDeleteResult` because it was redundant to `CompleteResult`.
* I improved the MergeVars tests and added tests for new methods.

**Slight reservations**

I would've liked to overload `GetMergeVars` so that it would accept a single list id.  I didn't do this because it did not seem to suit the established pattern.

I would've liked to create a `TryDeleteMergeVar` method.  I think it would be useful in cases where you weren't use a merge var existed, but I didn't want to hack an implementation.