Xcode project
Move Xcode project in Xcode/ directory

This is to use Xcode special config.h file in a independent directory.
The same is already done for MS Visual C with msvc/ directory.
Adapt Xcode project to the new paths
Add minimal config.h for Xcode

Xcode do not use ./configure so the config.h file needs to be generated
by hand.
This config.h is the minimal file for libusbx built using Xcode.
Xcode: remove implicit/default configuration
Xcode: build for 32 and 64 bits
make targets for examples depend on library target.  Generate library named libusb-1.0.0.dylib not libusbx.dylib.  Added several xcconfig files to make source control and documentation easier.  Reorganised a few files' positions within the project.  Disable strict aliasing since libusbx breaks strict aliasing rules anyway. Set project format as 3.1-compatible.

Conflicts:
	Xcode/libusbx.xcodeproj/project.pbxproj
fixed compiler warning about possible uninitialized use by zero initializing
fixed some clang -Wdocumentation warnings from bad doxygen markup
made some globals static to fix warnings
fixed link errors by adding missing files to Xcode project
darwin: fix clang warnings about explicit conversion

There shouldn't be any problems with any supported version of OSX in
converting a UInt64 to an unsigned long. They should be the same size
but even if they are not the session should still be unique.