marray_merge

I just found something that would be even more useful than a configurable delimiter: a 'merge' method. It currently takes a good bit of work to merge a normal array with a marray branch.

Clarification: It takes work because the values can't be returned by reference. So we must return to the old syntax in order to merge, defeating the class's purpose.

Further clarification: I hope which array will overwrite the other will be configurable via an optional argument, or the order of arguments as with 'array_merge'.

I'm not sure whether marrays should be mergeable, but I don't need that; I only need to put the root of one marray on the leaf of another.