Allow to reuse/set ZMQ context used by Mongrel2

[paneq]

As stated in: http://zguide.zeromq.org/page:all#Getting-the-Context-Right : "You should create and use exactly one context in your process"

Sometimes the programmer might want to create the Context itself and let
the handler use it. It can be done in two ways:

a) setting Mongrel2.zmq_context
b) providing it as a dependency when creating new Connection:

Mongrel2::Connection.new(sender_id, pull_addr, pub_addr, context)

But most people do not have such needs so by default we create our own
context with 1 thread for IO.

[paneq]

I also try to establish in this commit a basic culture of testing :) What do you think @perplexes , @pawelpacana ?

[mostlyobvious]

I totally support this since it:

• removes global CTX
• doesn't change default behaviour
• lazy loades context (which may be loaded lately i.e. after fork and won't cause trouble for those who need that behaviour)
• I can provide my own context when I need it

[mostlyobvious]

@paneq Can you please rebase on master?

[paneq]

yep.

[paneq]

Closing in favor of #12