Added license notice.
Show a warning message when tolua++ tool is missing.

For cross-compiling to be successful, all the tools must be already built natively.
Move tolua++ add_subdirectory instruction from main to LuaScript.

This should be part of commit 5dbf83c41819d789b758d07d5fcbf6a0272a309a.
Added documentation section on Lua garbage collection.
Merge remote-tracking branch 'remotes/szamq/master'
Re-added "flush GPU command buffer" -feature for controlling timestep variance and input lag. Direct3D only for now. Default off, as can decrease performance.
Refactor build scripts to build Urho3D library as main target.

Other changes in the commit:
- Change build option from URHO3D_BUILD_TYPE to URHO3D_LIB_TYPE. Default value is STATIC.
- Automatically add compiler definition 'URHO3D_STATIC_DEFINE' when linking with Urho3D library statically.
- Move LuaScript subsystem from Extra/ to Engine/.
- Enable FindUrho3D module to also detect Urho3D library target when the FindUrho3D module is being used internally by Urho3D project.
- Configure to build all the samples for iOS and Android platforms also when option to build samples is enabled.
- Export all the Register*Library() global functions.
- Rewrite all the tools, samples, and extras targets to link against Urho3D library.

TODOs:
- Combine engine sub-libraries into one.
- Test builds on non-Linux platforms.
Fix static lib build define.
Explicitly specify a base name for the export header generation.

The library target is renamed from "Urho3D" to "Urho3D_lib" as it clashes with "Urho3D" target for script host application. The library target's output name is then adjusted back "Urho3D" in the macro to generate "libUrho3D.a" or "libUrho3D.so" or "Urho3D.dll", etc. However, it seems that generate_export_header() macro provided by CMake does not taken into consideration of the adjusted target's output name property. Fortunately, it accepts a BASE_NAME argument to rectify the problem cleanly.
Fix Xcode AssetImpoter linker error caused by both freetype and assimp emitting same symbols.
Reverted back decision to also build samples on Android platform.

Although samples should also work for Android platform, currently there is no available mechanism to package each sample app into individual *.apk.
Added license notice.
Combine Engine sources into one place and build as a single target.
On a Visual Studio build, do not copy .obj files for the Urho3D library but use them from their original location.
Fixed ShaderCompiler build.
Fixed git revision generation on Win32 Android build.
Use one combined precompiled header file for Urho3D library, as otherwise precompiled headers would be disabled on Visual Studio.
Terminology edits in the documentation. Do not refer to Graphics, Scene etc. as separate libraries anymore.
Fixed Lua/LuaJIT compilation issue.

The combined Precompiled header does not include tolua++.h anymore, so it must be included back explicitly.
Simplify the build script to eliminate unused variable.

As the enable_pch() macro is not being called, there is no need to set SOURCE_FILES variable at this scope.