Add Enum.sum/1

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

Conflicts:
	lib/elixir/test/elixir/enum_test.exs
add ping method to Node module

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Update Node docs and CHANGELOG
Ensure do can be at the start of its own line, closes #2053
Do not allow structs on Dict nor Set
Add :app to dep opts