Project: Mac: add QMAKE_MAC_SDK

I got this error while compiling for OSX
   $ ~/Qt/5.3/clang_64/bin/qmake
   Project ERROR: Could not resolve SDK path for 'macosx10.8'

And check what version of SDK do I have, I found there are 10.9 and 10.10
   $ ls /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
   MacOSX10.10.sdk MacOSX10.9.sdk

I think we should all use the latest SDK, so I add this in project file
   QMAKE_MAC_SDK = macosx10.10

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>
project: rename mac to macx

The platform name "macx" is only for Mac OS X, however, "mac" is for both OSX and iOS.
And the settings in "mac" will overwrite the settings in "ios"
After corrected this platform setting, I'm sure that "ios" use the right Info.plist,
so we don't need that "NOTE" description in README.

   $ plutil -convert xml1 Debug-iphoneos/Terrarium.app/Info.plist
   $ grep -A1 CFBundleInfoDictionaryVersion Debug-iphoneos/Terrarium.app/Info.plist
	<key>CFBundleInfoDictionaryVersion</key>
	<string>6.0</string>

But this issue only happens when submitting app to App Store, so I didn't test it.
If I'm wrong, please reverst this commit.

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>
Merge pull request #10 from acelan/master

Add QMAKE_MAC_SDK and use macx