Support invoking iex/elixir for erjang

The commands `jex` and `jelixir` are symlinks to the corresponding `iex` and `elixir` scripts.

I'm totally open to discuss other options for how to do this; but this seams like the least impact solution. 

To use this, you need a `jerl` command in your path somewhere (make it a symbolic link to `ERJANG_HOME/jerl` )