Subscribers should parse & process using explicitly registered compatible types.

Before this change, when a 1_0 message is registered with a handler and/or
target class and a more recent minor version, 1_1, arrives at the subscriber:

- 1_1 may not be in the subscriber's namespace because it doesn't yet have
  an updated set of schemas. Result: subscriber drops a message it can
  parse and process.

- The subscriber has an updated set of schemas, but 1_1 is not registered
  with a custom handler. It can't find a custom handler for 1_1. Result:
  subscriber drops a message it can parse and process.

Here are the primary implications of this change:

- A 1_1 message written to the wire will now be handled by the 1_0
  message and the associated handler explicitly registered in a
  subscribing application. This will happen even if the 1_1 class isn't
  in the subscribing application's namespace.

- If the subscriber application wants to begin using changes in 1_1,
  then it has to explicitly register a handler against 1_1 instead of
  1_0. This prevents a message-parsing-compatible change to the schema
  from being processed by a handler that hasn't been built to handle
  the change in backwards-compatible way.

- We will *always* parse a later minor version with the minor version
  registered within the subscriber application. This is possible
  as long as we use Protocol Buffer schema evolution rules here:
  https://developers.google.com/protocol-buffers/docs/proto#updating.
  The registered minor version is the object that will be passed
  to its associated registered handler.

When a message comes in we now do the following:

1) We find a compatible, explicitly registered message type (e.g. the
   same base type on the same major version)
2) Use that type to both parse the payload and pass to its registered
   handler.
Test cases around handle_envelope

Breaking out test cased for the following scenarios:
* when there is no compatible version registered for the message
* when there is a compatible version registered for the message
Removing order dependent nature of test

In the previous  commit, setting test_emitter caused an issue of the
emitter being set for other cases using the instance variable depending
on the other that the test cases run
Merge pull request #8 from harrystech/find_compatible_handler

A patch to find compatible handler