optional single-line comment translation and spaces around operators

Adds optional single-line comment conversion from `MATLAB` to `python`. The command switch `-n` (for [`--no-comments`](https://github.com/johnyf/smop/compare/victorlei:master...master#diff-80f9821cc9bc32f4b9d4cc3df91d17ebR18) strips comments. This update is working but not tested thoroughly.

Multi-line comments are not supported yet.

Single spaces have been added around binary operators, single spaces after commas.

A single space precedes `+` and `-` but no space follows (this is not constrained by `PEP8`), by personal preference regarding readability, treating them more like increments in a sequence of additions than binary operators (since this is infix and not prefix notation).