Expose logger through api operation base

[HeathNaylor]

Problem:
There is currently no way to set the log level of the logger class

Solution:
Expose the logger via a getter on the API Operation Base class. Any class extending this (Controllers) will now have access to the logger and be able to invoke $controller->getLogger()->setLogLevel(Log::ANET_LOG_WARN);

Notable changes:

• All globally defined variables have been moved into the class scope as public constants. This will prevent implementers of this library from defining the global variables and raising PHP notices. This also allows for any IDE to pick up on the public visibility and give the constants as recommended selections.
• $logger should never be null

Fixes #186