adds senders to database and firebase connect

- senders, which now have a firebase token
  are added as their own table in the database
- firebase token api call added, which allows
  a user to connect a sender id to a firebase token
- firebase library not yet connected to the saved
  firebase tokens

lib/bump/senders/sender.ex

@@ -0,0 +1,18 @@
+defmodule Bump.Senders.Sender do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  schema "senders" do
+    field :sender, :string
+    field :firebase_token, :string, default: ""
+    timestamps()
+  end
+
+  def changeset(sender, params) do
+   sender 
+    |> cast(params, [:sender, :firebase_token])
+  end
+end
+
+
+