implement object[Integer...]

Maybe make `object[i,j,...]` in Julia sugar for `get(object, (i,j,...))`, equivalent to `object[i,j,...]` in Python, for the common case of integer indexing (e.g. #149).

I would prefer to just make `object[...]` equivalent to `object[...]` in Python regardless of the index type, but then I wouldn't have a good syntax for Python's `object.attribute` (until some future Nirvana when dot overloading arrives in Julia).

Review the proposed object[i,j,...] behavior and the referenced issue #149. Determine whether integer indexing should map to get(object, (i,j,...)) and how the proposal relates to Python-style indexing and attribute access. Done means the desired syntax and its scope are agreed and implemented consistently.

索引モデルが issue の本文から書いたものです。