scapi could not compile on osx mavericks before since in mavericks
apple moved to the clang compiler (based on llvm) whether the makefile
is built on the assumption that the compiler is gcc.

in order to maintain a unified makefile for both linux and mac os x,
there is a need to manually specify gcc-4.9 as the compiler (only for mavericks users).

this is now handled automatically in the makefile, but the user is required to install
gcc49 via homebrew. this will be addressed in the online documentaion.
updated the installation instructions to include the requirements on OS X Mavericks and above.
forgot to add proper linking to clang and homebrew in the install doc
Merge pull request #18 from cryptobiu/compile_on_osx_mavericks

Fixed makefile that works on OS X Mavericks and updated install doc