Add spread event stream for all exchanges

[bmancini55]

Some exchanges implement a "spread" or "top of book" stream (see conversation in #142 (comment)). This issue will create a separate event stream for exchanges that support this type of stream. Complexity surrounding the ticker event including "top of book" changes will be deferred in this issue. We will make appropriate notes in the README on exchange functionality where "top of book" functionality is mixed with "ticker" functionality.

• create event type
• add event support to base client
• add event support to multi client
• update README with event information
• update exchanges appropriately