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

This fixes issue #17 .

In OS X 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 is addressed in the online documentaion.