Update.

- Fixed install directory for include files.
- Made "make install" dependent on "make test".
- Added "const" where possible.
- Added extensions to <mx>.
- Added support for passing NULL pointers to strunpack.
- Streamlined order of payload and payload size arguments in mx.c
- Added tests for NULL pointers in bufUnpack.
- Tweaks to README.
- Added some necessary includes to mx.h.