Revert changes to OS X README for 10.6, it does not work anymore.

This reverts commit 6de983bcd8cf12664767fb825fd3d1f910b404c9.
This reverts commit cfba89ffbce1e64e89f55fae8f4881d4473c037a.
Improve the OS X ReadMe
Merge pull request #143 from eregon/osx10.6

Update Mac OS X's ReadMe
Found and fixed problem with recent GTest revisions

* __FLOAT_H is defined both by our vm/vm/main/float.hh
  and Clang's float.h (usually /usr/lib/clang/5.0/include/float.h).
* It might also be considered as a problem of Clang
  (other #define have weird names or start with __CLANG_).
Merge pull request #146 from eregon/bad_define

Found and fixed problem with recent GTest revisions
Fix include guard naming scheme

* See https://github.com/mozart/mozart2/pull/146
* The usage of leading __ is reserved to the implementation.
Merge pull request #147 from eregon/fix_include_guard_names

Fix include guard naming scheme
Removed most of Darwin-specific configuration.

* Pass CMAKE_CXX_FLAGS to cmake subprocesses instead.
Raise an error when small integer overflow happens.
Move Float methods to the FloatLike interface

* So not all numeric types must implement them.
* Not sure where divide (/) should be defined,
  but it only makes sense on floats so I went with moving it as well.
Implement Number.pow in Oz, so it calls the right multiplication operator

* This will make the transition to BigInt easier
* The code added in Number.oz is taken from Mozart 1
* I named the float power operator Pow (was fPow in Mozart 1)
Remove virtual from BoostBasedVM::newBigInt, it should not be overridden.
Start the foundations for big integers.

* Architecture: BigInt -> BigIntImplem (abstract, interface)
                       -> BoostBigInt (which will contain an actual boost multi precision integer).
* Using shared_ptr ensures automatic deallocation.
* The VM argument might not be needed in VirtualMachineEnvironment::newBigIntImplem
  but right now it is to raise an exception in the default VMEnv.
  That method should of course never be called in the default VMEnv
  (only from BigInt when we managed to call newBigInt()).
* We need to shrink BigInt to SmallInt.
Implement BigInt multiplication

* For now, using cpp_int for simplicity
Avoid creating two BoostBigInt when only one is possible.

* std::make_shared() will call the constructor,
  so pass a suitable argument for BoostBigInt constructors.
Make BoostBigInt::make_shared_ptr generic in its argument.
Ensure to not copy twice the BoostBigInt constructor mp_int argument.
The Boost Multiprecision library is new from Boost 1.53.0

* 1.53.0 was released on February 4th, 2013.
* We need it for big integers.
Implement BigInt::equals()
Implement BigInt addition