WIP: Fix backslash-indentation and dotted-operator indentation

[non-Jedi]

Closes #58. Closes #56.

There may be a better way of handling dotted operators, but I went with matching what was already there.

[non-Jedi]

I missed the bit where \ has to be an escape char for string escaping to work correctly. For reference of how to do this correctly see octave-syntax-propertize-sqs. Don't have time to fix this evening.

[non-Jedi]

The fix here is dependent on dropping Emacs 23 support (#86). Not going to implement twice, once with syntax-propertize-function and once without.

[tpapp]

This is fine, I will just wait a month for feedback before merging #86.

[non-Jedi]

This is going to end up looking entirely different when I get around to it. Will open a new PR.

[tpapp]

Great, thanks.