Allow global and per process configuration

Hi, following back on #7 i've finished the improvement of ExTwitter configuration to allow global and per-proccess configuration.

This also removes the use of an ETS table in favor of Elixir's Application `get_env`/`set_env`, the per-process configuration uses the Process dictionary. The `ExTwitter.Config` API remains unchanged for global configuration.

To configure a single process you'll need to call `ExTwitter.configure(:process, [consumer_key: "", ..."])`.