Implement and provide OSX staticly linked binaries

Spin-off of #156.

Most mac users won't have a development environment or the skills to compile aria2 themselves. While aria2 is provided by most *nix package managers, and there are official Windows and Android build, there is no good way to obtain binaries for OSX.

Unfortunately, it is quite hard to get an OSX toolchain working on a non-OSX platform.

 - [x] Create first static builds
 - [x] Universal binaries
 - [x] Installable pkg in a dmg