DDLSHELL-25: Add plugin SPI

Adds new unit tests; tested with mvn verify.
In the middle of this series of commits it adds a module named "aaron" which responds to the single command `meep;`. I tested this with the following sequence on a bento box:

    bin/kiji-schema-shell
    
    SHOW MODULES;
    MODULE aaron;
    meep;
    quit;

The `meep` command is equivalent to `SHOW INSTANCES;` in its effect, but does not work unless you explicitly load it through `MODULE aaron;`.

The final commit in the series removes the test module.
These should be squashed before pushing.