Allow specifying api_version and SFDC endpoint

Prior to this commit, library users were not able to customize which
version of the API they used, (at least not prior to login).
Additionally, the library offered no way to log into a test or sandbox
instance of SalesForce. This commit allows library users to create
their own Forcex.Client structs as a starting point. This will allow
using different API versions and sandbox instances. This may also allow
different authentication grant_types other than password.

Close #7.

README.md

@@ -91,6 +91,20 @@ first_page = Forcex.query("select Id, Name from Account order by CreatedDate des
 second_page = first_page |> Map.get("nextRecordsUrl") |> Forcex.get(client)
 ```
 
+## Further Configuration
+
+Forcex allows additional configuration of API endpoint and API version via the
+`%Forcex.Client{}` struct. You may also use this mechanism if you have a
+`grant_type` other than password.
+
+This example shows how to use both an older API version and the SalesForce
+sandbox API.
+```elixir
+Forcex.Client.default_config
+|> Forcex.Client.login(%Forcex.Client{endpoint: "https://test.salesforce.com", api_version: "34.0"})
+```
+
+
 ## Current State
 
 See https://www.salesforce.com/us/developer/docs/api_rest/

lib/forcex/client.ex

@@ -44,12 +44,16 @@ defmodule Forcex.Client do
         Forcex.Client.login
         |> Forcex.Client.locate_services
   """
-  def login(c \\ config) do
+  def login(c \\ default_config) do
+    login(c, %__MODULE__{})
+  end
+
+  def login(conf, starting_struct) do
     login_payload =
-      c
-      |> Map.put(:password, "#{c.password}#{c.security_token}")
+      conf
+      |> Map.put(:password, "#{conf.password}#{conf.security_token}")
       |> Map.put(:grant_type, "password")
-    Forcex.post("/services/oauth2/token?#{URI.encode_query(login_payload)}", %__MODULE__{})
+    Forcex.post("/services/oauth2/token?#{URI.encode_query(login_payload)}", starting_struct)
     |> handle_login_response
   end
 
@@ -66,7 +70,7 @@ defmodule Forcex.Client do
     %__MODULE__{}
   end
 
-  defp config do
+  def default_config do
     [:username, :password, :security_token, :client_id, :client_secret]
     |> Enum.map(&( {&1, get_val_from_env(&1)}))
     |> Enum.into(%{})

mix.exs

@@ -8,7 +8,7 @@ defmodule Forcex.Mixfile do
   def project do
     [
       app: :forcex,
-      version: "0.4.0",
+      version: "0.4.1",
       elixir: "~> 1.0",
       name: "Forcex",
       description: @description,