Norwegian (bokmål) translation

Language code: nb_no 
Language name: Norsk (bokmål)

Norwegian has two written languages, the other being "nynorsk", thus the split language code. This actually turns out to be fortuitous, as "no" isn't a valid code for the top level element in YAML (it parses as "false"). :)

I ran checker.py on it, no errors reported.