Added GetAssemblyVersion helper

As discussed in #422 (used @mexx method as it seems superior to what I was proposing). Introduced AssemblyHelper (created new helper, as I didn't want to touch other ones, although this name might not be the best?). 

Test might be a little bit to simple, because there's no guarantee that version get from calling assembly will have every 4 fields in versions different, but creating new temp assembly seemed as an overkill.