Add support for loadable MesosModule

I think we should break this into multiple phases.
-(1) Let's get the dynamic library loading via a "stout-ified" version of https://github.com/timothysc/tests/blob/master/plugin_modules/DynamicLibrary.h. -
DONE

(2) Use (1) to instantiate some classes in Mesos (like an Authenticator and/or isolator) from a dynamic library. This will give us some more experience with how we want to name the underlying library symbol, how we want to specify flags for finding the library, what types of validation we want when loading a library.
TARGET

(3) After doing (2) for one or two classes in Mesos I think we can formalize the approach in a "mesos-ified" version of https://github.com/timothysc/tests/blob/master/plugin_modules/MesosModule.h.
NEXT