Initial client impl.

A simple (non-pooling) HTTP client for the TrakIO REST API.

The `GenServer` client should be supervised when adding this project to an application. To start it without supervision call `Pathway.Client.start_link` before making any requests.

You can configure the client in your `config.exs` file by adding the following lines:

```elixir
config :pathway,
  apikey: "some apikey",
  timeout: 3000 # default
  retries: 2 # default
```

If the `timeout` or `retries` application keys are not configured they default to the values shown above. You __must__ configure an API key.

An example TrakIO API request looks like: `Pathway.annotate("some global event")`.