Added some C++14 TODOs.
Make explicit `std::hash` specialization for ResourceKey.

Allows users to use ResourceKey in `std::unordered_map` more
conveniently.
Fix example code in documentation, added TODO.
Make virtual AbstractResourceLoader functions private.

Removes the need to call AbstractResourceLoader::load() from
reimplementation.
Test whole AbstractResourceLoader API.
Simplified internal ResourceManager::free() implementation.

No need to explicitly create all the pointers and then juggle with
`std::common_type<>` and whatnot.
Deleting all resource loaders before unloading any resource data.

Previously was done per type (i.e. for each type delete loader and then
the data), wasn't usable when the loader stored different resource type
(which is the only case where it is useful).
Simplified ResourceManager::set() convenience function.
Added convience AbstractResourceLoader::set() overload.

Similar to ResourceManager::set() one.
Minor cleanup in ResourceManagerTest.
Ability to force cleanup of ResourceManager.

Assumes that no resources of given type are referenced. The test case
for this is awful, as I can't think of a way to test which would work
without trying to decrement reference counter on already deleted
resource. Added just-to-be-sure assertion to make it fail early and not
somewhere deep in STL.
Trade: use instance() instead of instanceId() for consistency.

Nowhere else is `Id` suffix.
Trade: improved documentation, removed redundancy.
Constexpr Array from one value constructor, improved test.

Merged constexpr test cases into other ones to remove duplicate code.
Copied the constructors from Math::Vector as I don't feel reinventing
all that again.
Various Array-related cleanup.
Ability to retrieve first two components from Array3D.
MeshTools: added TODO.
Minor documentation fix.
Minor cleanup.
Added TODO.