some changes to follow best practices

Addressing the issues raised in #4. 

@thejameskyle do you mind to take a quick look? There are two remaining issues:

- watchify doesn't seem to get triggered for changes
- the watchify code duplicates much of the regular build task code

For the second one, I haven't really looked into it and this is the first time ever that I'm using gulp. I'm happy to dig into the rabbit hole later myself but feel free to leave a hint if you like :)

However, do you know from the top of your head what's wrong with the watchify task?

/cc @PascalPrecht