Refactor build scripts.

- Close issue #67, Urho3D iOS library should be built correctly now (either as single arch or as multiple archs Mach-O universal binary). Urho3D library build for Mac OS X and iOS platforms do not rely on CMake hack 'object-collecting' approach anymore. It uses Apple static linker 'ld' tool to merge all objects from third-party *.a directly.
- Add new custom target (Urho3D_universal) for iOS platform to build a single Mach-O universal binary library consists of both iphoneos (universal ARM archs) and iphonesimulator (i386 arch).
- Introduce PLATFORM_PREFIX (ios-, raspi-, android-) to prefix the build and output directories (Build, Bin, Lib).
- Correct Xcode-specific build setting for Mac OS X desktop build to use the latest OS X SDK but set deployment target to current OS X of the build system, unless CMAKE_OSX_DEPLOYMENT_TARGET is set explicitly to build for other target.
- Temporary workaround for CMake/Xcode generator bug where it always appends '/build' path element to SYMROOT attribute and as such the items in Products are always rendered as red as if they are not yet built.
- Change Assimp target to use setup_library() macro so that it gets the same settings (and workaround) as all other targets.
- Add a conditional check to prevent MSVC to build 'buildvm-android' target as it is not supported at this moment.