Option to not produce images

I use NIR spectra for soil modelling and I really like your package as it allows easy reading in of asd files in R. However, I usually batch a few thousand spectra at a time. Most of the time, I'm not really interested in the graphs, just in getting the spectra converted.

I made a couple of changes in the `read.asd` function and I'm presenting them here as a pull request.

Major:
- Added new parameter making images optional
- Function now returns data.frame invisibly

Minor:
- Converted warnings to be more compact
- Added spacing after punctuation