[WIP] Octave/Matlab codegen and code printing

I'm working on Octave/Matlab code printing and codegen support.

TODO:

 - [X] Octave code printer (not complete)
 - [X] Octave codegen (not complete)
 - [x] Lists
 - [x] Matrices
 - [X] Logical ops
 - [ ] Logical, `True`/`False`
 - [x] Tests, based on C/Fortran tests.
 - [x] Piecewise support
 - [ ] Comment headers to appear after the function prototype line.
 - [ ] Use vectorized elementwise operations, e.g., `.*`, `.^`, (unless we know a Matrix is involved?)
 - [ ] Indenting
 - [ ] deal gracefully like C/Fortran with "not supported"...
 - [X] Is superclass `CodeGen` a bit too C/Fortran specific?  mathematica.py
       was a bit thinner, but is for code printing, not codegen.  Seems it will be ok, especially after @jcrist's recent changes.
 - [ ] Single-statement/single line support?  This is important for generating "handle functions", e.g., [https://github.com/cbm755/octsympy/blob/master/inst/%40sym/matlabFunction.m] as opposed to .m files.  Currently controlled by whether there is `Assignment` or not.  Think more about this.

Comments/help very welcome!
Shouldn't all of these default to the dot operators? i.e. `.^` instead of `^`?