Add has_keys? to Keyword

This pull request add a function to answer if a list of keys are defined in a Keyword list.

This is useful when you need to know if a list of properties is defined, for example on a keyword configuration list:

```elixir
if Keyword.has_keys?(config, [:facebook_appid, :facebook_secret]), do: something(config),
else: log(:error, "Configuration)
```