Create spectroscopy-letters

I've started working from the applied spectroscopy style as mentioned in the summary. 
There are still a few issues, but at least it seems to be compliant to 100% with journal references
Issue: a comma after the date still appears in the book references and I didn't know how to remove it. Probably it works with conditionals...)
Yeah, but we should try to make it clean,i.e. delete some errornous commas. You can have a look at https://gist.github.com/d6489bacb9e60542acb9.git for the comma issue. I deleted the comma and updated the lines 183ff. You can just copy the relevant parts back your location.