Fix a bug in insert_at adding items to the wrong location with negative indexes
Add Enum.reduce/2 that sets initial value of acc to the first element of collection

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Conflicts:
	lib/elixir/lib/enum.ex
Refactor Enum.min/1 and Enum.max/1
Deprecate Enum.min/2 and Enum.max/2 in favor of Enum.min_by/2 and Enum.max_by/2
Update CHANGELOG