Rename json functions away from +pkg syntax.
Use Octave-compatible syntax for calling Java.
Add support for invoking Octave instead of Matlab.

* Extract most of the Matlab class into a base Session class
* Matlab and Octave are subclasses that tweak the command line
  invocation
* Eliminate some code duplication along the way
s/Session/_Session/, and restore its docstring
Use appropriate program name in output.
Make test suite runnable with Octave.
Fix messenger compile errors on gcc / clang.
Mention Octave support in README.
Merge pull request #75 from isbadawi/octave

Octave support