Added Mac-specific building flags

'GCC_ENABLE_CPP_EXCEPTIONS': 'YES'
fixes this compilation error:
error: cannot use 'throw' with exceptions disabled

'GCC_ENABLE_CPP_RTTI': 'YES'
fixes this compilation error:
cannot use typeid with -fno-rtti" issue

"MACOSX_DEPLOYMENT_TARGET": "10.7",
"OTHER_CPLUSPLUSFLAGS": [
    "-stdlib=libc++"
]
fixes a runtime linking error. here is a sample message that was displayed:
dyld: lazy symbol binding failed: Symbol not found
Merge pull request #2 from PaulMougel/fix-gyp-mac

Added Mac-specific building flags