Added an implicit parameter for the ExecutionContext

[breckcs]

• Rather than importing the global ExecutionContext, allow the caller to
  specify the ExecutionContext.
• Changed the minor version to 1.2.0 since this is a breaking change to the
  API. All the caller needs to do to maintain the existing behaviour, however,
  is import the global ExecutionContext, if it has not already been imported,
  as evidenced by the changes to the tests in this commit.

[mkiedys]

I feel that we can retain compatibility while also allowing people to pass their own execution context. Akka faced similar problem, and they way they are doing it is quite good. Can you look at ActorSystem companion object?

We could still use default shared thread pool of two by default that gets disposed after last connection is closed but also allow people to pass their own ExecutionContext that they control and shutdown after it is no longer needed. It is both more convenient and most likely more performant solution to have some sane defaults.