Handle int input

Update renderer.core.Prompt.apply_default to convert given values to
strings in case someone happens to provide a literal int
Add compat layer for string checking

Update compat to include string_types for use in determining if an
object is a string in python 2 or python 3
Enable list aggregation for duplicate input keys

Update Form when a dict conversion is taking place to store duplicate
keys in a list. If 2 inputs share the same key, their values will be
returned as a list in the order the prompts were presented.

Add corresponding test