style guide

[Jesse Meek]

I just proposed a PR for a style guide: 
https://github.com/juju/juju/pull/250

Below is the justification:

"First pass at a style guide. Most of this *should* be common sense. 
Yet, as everything listed here frequently comes up in code reviews, 
there is arguably a need to make common sense a little more common.

I expect there will be some debate over what is a juju convention/style 
vs someone's opinion. As such, I provide this doc only as a starting 
point and I am not attached to any given convention in there. The 
benefits of agreeing on and clarifying our conventions upfront are: 
easier 'on-ramping' for newbies and quicker reviews.

I will start a juju-dev thread for this PR, please use it for debating 
what should/shouldn't be in the style guide (or even if you disagree 
that we need a guide)."