Improve documentation

- README updated
- New dummy .m files with help to .mex files
- Some issues with `dir` and `ls` solved

TODO:
- Solve strange behaviour with `ZMQ_MAXMSGSIZE`, seems to be unsigned
- Implement 'ZMQ_TYPE' as string
- Externalize 'pointer_from_m'
- Add Travis support

I think my editor touched line endings... this could be a problem?