Record base_url returned from successful login

Per Docusign team: "In the demo environment all account URLs
start with demo.docusign.net, however in production there are
multiple sub-domains like www, na2, eu, etc.  To resolve all you need to
do is parse the baseUrl value that is returned from the login call."

This seems to also apply to the oauth token, but I'm not sure of the use
case there.

Thanks to Will Beattie for the report.

CHANGELOG.md

@@ -1,9 +1,10 @@
 # Changelog
 
-## v0.2.1 April ?? 2017
+## v0.2.1 May ?? 2017
 
 ### Features:
 * Add certificate option to DocusignRest::Client#get_combined_document_from_envelope (Shane Stanford)
+* Memoize base_url after successful login attempt.  This prevents problems in production when Docusign returns a different subdomain on login as discussed in [here](https://github.com/jondkinney/docusign_rest/pull/102) (Tom Copeland)
 
 ### Misc:
 * Replace monkeypatch with argument usage (Jean-Philippe Moal)

lib/docusign_rest/client.rb

@@ -6,7 +6,7 @@ module DocusignRest
   class Client
     # Define the same set of accessors as the DocusignRest module
     attr_accessor *Configuration::VALID_CONFIG_KEYS
-    attr_accessor :docusign_authentication_headers, :acct_id
+    attr_accessor :docusign_authentication_headers, :acct_id, :base_url
 
     def initialize(options={})
       # Merge the config values from the module and those passed to the client.
@@ -80,7 +80,12 @@ def headers(user_defined_headers={})
     #
     # Returns a parsed URI object
     def build_uri(url)
-      URI.parse("#{endpoint}/#{api_version}#{url}")
+      uri_string = if base_url
+        "#{base_url}#{url}"
+      else
+        "#{endpoint}/#{api_version}#{url}"
+      end
+      URI.parse(uri_string)
     end
 
 
@@ -190,6 +195,7 @@ def get_account_id
         hashed_response = JSON.parse(response)
         login_accounts = hashed_response['loginAccounts']
         @acct_id ||= login_accounts.first['accountId']
+        @base_url = login_accounts.first['baseUrl']
       end
 
       acct_id