Smaller modification of pull request #24 - compiling issues on OSX

Change needed for compilation with gcc-6

On OS X the file system usually is case-insensitive, hence installing this script will overide the executable `music`.

-Wl,-z,defs is not supported on OS X

defs
Disallows undefined symbols in object files. Undefined symbols in shared libraries are still allowed.

removing it will not hamper performance of music

Python 3 Port

Fixed wrong conversion from pickled bytearray to str in MessageCallback. The pickle protocol adds some meta information bytes in front of the actual string, so a conversion to Cython str is invalid in general.

Missing virtual destructors.

Modified the .travis .yml file : modified the download location of get-pip, using minimal image of trusty, installing python-all-dev

Add a check for mpi4py version during Cython compilation.

Version 2 of mpi4py has changed the name of a submodule from mpi_c to
libmpi. We need to check which one to use at compile time.