fix(*): add godocs and address other golint issues

[arschles]

Also:

• Ensure that comments (including godoc comments, but not code) don't expand beyond the 99 character limit described in the code standards
• Fix some small grammatical issues in docs
• Ensure that all godoc documentation is written with complete sentences

Fixes #230

[mboersma]

Awesome! Two minor comments:

• Our code standards limit Go code width to 99 chars, but some of these doc comments look longer.
• Effective Go recommends complete sentences, so these doc comments should end with periods like in Go's stdlib.

[arschles]

@mboersma thanks for the comments!

I'll modify this code to adhere to the 99 character limit, and I'll also ensure that we have complete sentences, with punctuation, in our docs. Commits forthcoming.

[arschles]

@mboersma 4b534587f96cf9b8e7b79a110f288520485a8819 and 3aa2eb408e7a889b67ac56312450da8de417290b limit comment lines (godoc and otherwise) to 99 chars and ensure that sentences are properly terminated.

Also, sidenote: I've created #251 to capture the work for writing package-level godocs. I'm considering that out of scope of this PR.

[mboersma]

There were already a lot of good documentation comments in here, and this improves many of them and fills in gaps where they were missing. This is excellent-I re-read this whole PR and I feel like I have a better understanding of builder just from that. :+1:

[arschles]

thanks @mboersma

fwiw, things will get even clearer after https://github.com/deis/builder/issues/152 is done, but I digress.... :see_no_evil:

[arschles]

Merged as part of v2.0-beta1, since I believe this will help new contributors to get started on the codebase