fixes OSX installation error (now compatible with XCode 5.1 + clang compiler)

Here's the situation:  
* distutils automatically generates `-mno-fused-madd` as a clang compiler option for python versions that are preinstalled on OSX
* but `mno-fused-madd` is not a recognized option in newer versions of the clang compiler (e.g., the version I was using was "(clang-503.0.38) (based on LLVM 3.4svn)")
* and Xcode 5.1 now treats unrecognized compiler options as errors, not warnings (see [the compiler section here](https://developer.apple.com/library/ios/releasenotes/DeveloperTools/RN-Xcode/Introduction/Introduction.html))
* since scrapy relies on distutils (if setuptools is missing doesn't work), installation using distutils was failing because of this unrecognized option

To solve this, I grabbed some code from [PyMongo's setup.py script](https://github.com/mongodb/mongo-python-driver/blob/master/setup.py) that addresses this by removing "-mno-fused-madd from the CFLAGS automatically generated by distutils for Apple provided pythons" and put it in scrapy's setup.py script, which fixed the installation issue.