Fix serialization handler registration by priority

[discordier]

This fixes #656 by examining the priorities for each direction+type+format combination and not globally.

This now allows to register the same handler for multiple types and/or formats as it was possible in pre 2.4 versions.

[discordier]

Build failure related to travis outage, the PHP 5.6 build hang for more than 10 mins.

@goetas could you restart the build in travis please?

[goetas]

@discordier this is an alternative implementation that solves the problem without a special case for subscribers and listeners and not concatenation.

(but I have included your tests)

As we are migrating the bundle to MIT, do you agree to license your changes under the MIT license?

[discordier]

I do, I guess I already denoted so in the license change issue.