Rework merge signature

Given dictionaries `dict`, `dict2` and `dict3` previously merge was
called via:
$.merge(dict, dictionaries: dict2, dict3)

With these changes merge becomes:
$.merge(dictionaries: dict, dict2, dict3)

and gains
$.merge(arrays: arr, arr2, arr3)
Merge pull request #31 from zolrath/merge-signature

Rework merge signature