Minor cleanup - remove redundant null check for subsystems pointer.

Some of the subsystems are registered when the Engine is instantiated and I believe their references should remain valid until the engine stops.
Minor enhancement on getting optional resources and error handling.
Prevent any usage of windowless MessageBox.
Enhance editor to pop open the console when receiving error events.
Travis CI: API documentation update at 2014-02-10 00:07:21 UTC.
[ci skip]
Code cleanup on error handling for ResourceCache.
Travis CI: API documentation update at 2014-02-10 06:08:42 UTC.
[ci skip]
Do not force RigidBody's interpolation transform when the body is kinematic to allow velocity estimation to work right for dynamic bodies colliding with the kinematic body. Thanks to Andy51.
Changed WorkQueue to use a List<SharedPtr<WorkItem> > and ensure that it doesn't contain duplicates.
Changed AddWorkItem to take in a SharedPtr<WorkItem>.
Changed Render functions to use new WorkItem design.
Better fix for kinematic rigidbodies: let Bullet ask the world transform (as intended) from the scene node instead of us forcing it. You can still force rigidbody position using RigidBody::SetPosition() but it should be unnecessary.
Added automatic extension when saving material as
Merge remote-tracking branch 'remotes/alexparlett/feature/work-item-refactor'
Bullet rigidbody's inertia tensor does not depend on position, so no need to recalculate it if rigidbody position only is forced to a new value.
Fix order of exposing AnimatedModel functions to Lua so that all overloads of SetMorphWeight() & GetMorphWeight() work. Closes #182.
Travis CI: API documentation update at 2014-02-11 01:34:00 UTC.
[ci skip]
Minor tweak to link lua/luajit tool without Urho3D dependency libs.

These changes should also reduce prerequisite for building 32-bit luaJIT VM (for Android and RASPI) on 64-bit host/build system.
Added drag and tooltip to sample#2 HelloGUI
Fix various order of exposition to lua
For Travis CI - attempt to setup 32-bit and Android CI build.
For Travis CI - another attempt to add more CI build matrix.