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.
Bump required CMake version.
Do not use PCH on AssImp contrib source files, as they're not all prepared for that.
Use constant for number of draw buffers.
Add a grid to the editor. Add default zone and grid editor preferences. Make editor settings and preferences dialogs resizable and scrollable. Fix Pow() script registration.
Use -force_load instead of -all_load option for Apple Static linker.

It is to ensure only objects from the specific static libraries are being merged into final output. Objects from standard libraries (ld, m, etc) should be excluded.
Fixed GLES low quality shadows. Fixed Android/iOS not initializing engine parameters from CommandLine.txt.
Added ability to specify window icon (external application window) using an Image resource and SDL cross platform capability
Added Script Bindings in Lua and Angelscript to support the function
Added functionality to set the value for a node in the form of <node>value</node>.
Added method to retrieve the value of a node held in the inner xml tags eg. <node>value</node> would return "value" when called on "node"
Added Script Methods for Getting and Setting the value as detailed above (Lua contains Getter only inline with the rest of the functionality)
Removed OpenGL scratch buffer allocation log spam, even in debug mode.
Merge remote-tracking branch 'remotes/alexparlett/window-icon'
Merge remote-tracking branch 'remotes/alexparlett/xml-value'
Normalize whitespace, update documentation.
Added AngelScript & Lua property accessors for the XMLElement's value.
Added setters for Lua XMLElement.
Updated scripting documentation.
Added Image::GetSDLSurface() to avoid code duplication. Fixed crash from null image pointer in Graphics::CreateWindowIcon().
Fixed XMLElement on GCC.
Fix function order in header.
Fix Mac OS X and iOS build from external CMake project.

Since the CMake minimum is bumped up, set the policy CMP0022 to NEW. Incidentally it is also required to be so to stop the static libraries dependency from being transitively get included to other targets that links to Urho3D shared library.
Clean up scene node related comments in samples.
Use CMake command to move and rename for portability. Closes #73.

- Revert back CMake version. Use CMake -Wno-dev option to suppress the policy warning for CMake 2.8.12 early adopter.
- Since the new policy is not in place, set some more target property to prevent Urho3D static dependency libs from being transitively link to other targets linking to Urho3D.