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...)
I added edit instructions are on the bottom. We'll check again before merging, you can't place the style in the public repository yourself, so no worries. (And github is the code hosting site, they don't do anything; the people reviewing this do CSL exclusively).