Message sending and Sent Message reading enhancements

**Improvements**
* Account.send_message/Account.send_messages now returns DispatcherResult providing array of submission hashes (*messages*) { :id, :uri }
* DispatcherResult.to_s returns *batch_id* to retain behaviour of existing string return value

**Additions**
* Account.sent_messages returns SentMessagesResult containing messages sent via Account's *reference*
* SentMessagesClient.get_messages returns SentMessagesResult containing messages sent via Accounts accessible by the authenticated user
* SentMessagesClient.get_messages provides filtering by *account_reference*, windowing by *start* and *finish* dates, and paging by *start_index* and *count*
* Account.sent_messages accepts same criteria as SentMessagesClient except *account_reference* will always be overridden to the Account's *reference* and date windowing will default either *finish* or *start* date if either is missing when the other is given
* SentMessagesResult provides *next_page* & *previous_page* navigation methods to move between pages using same criteria as used to create initial result
* SentMessagesResult provides array of SentMessage (*messages*), *page*, *total_pages* and *total_messages* attributes together with *last_page?* and *first_page?* helpers
* SentMessage will automatically retrieve and cache full message body if not already available via *summary* attribute