-Make CEREAL_THREAD_SAFE be 0 or 1 instead of present or not present
-Move CEREAL_NOEXCEPT to macros.hpp
-instead of individual locks, can now use StaticObject::lock() to request a lock if CEREAL_THREAD_SAFE is enabled. If
not enabled, this call returns an empty object. The lock returned acts just like std::lock_guard, but uses
std::unique_lock internally
-Made a bunch of requests to StaticObject::getInstance const
-Added first stab at multithreaded tests with versioning and polymorphism
relates to #281
Now if someone has a hierarchy like A -> B -> C,
the A->C relation will be automatically created if A->B and B->C already exist.
This is implemented by a chain of casts to get down the inheritance.
In the case of multiple paths, the shortest one will be chosen.
