adding option to create nested lists on the fly

[martinResearch]

adding option integer_as_list_index to dpath.util.new to tell it to interpret integers as list indices so that new lists are automatically created if needed when encounting integers. This is very convenient to create nested lists (feature request #91). This also fix the bug #92

[akesterson]

Thanks for the suggestion. Ultimately we found that we wanted to avoid adding complexity to dpath core to handle what essentially boiled down to a given user's specific use case governed by their app logic. Instead we went with the approach in #101. .Please review this and let us know f that will be unable to handle your use case.