dgtal 0.8.0

DGtal upgrade to 0.8.0 and formula cleanup
I was talking to Misty about this on IRC and she said: "Looking at it, dgtal is a is a library - so I guess C++11 seems like it should be opt-in if you're not using clang on a platform where libc++ is the default, to prevent stdlib incompatibility issues?"

So it's fine to build with C++11 with clang on Mavericks and above (`ENV.compiler == :clang && MacOS.version >= :mavericks`) but it should be off or optional otherwise, I think. `option :cxx11` will add a --c++11 option to the formula.