Forget plot on `drawImage`

MATLAB cannot add a legend entry for images without doing
some manual legend manipulations, which is not supported in `matlab2tikz`
anyhow.

`pgfplots` on the other hand does support legend entries for these,
without any automatic effects, though. The sensible thing to do, is
to just turn off this behavior in `pgfplots`.
`drawImage`: using `addToOptions`
Merge branch 'fix295'