This updates GofmtTool to subclass the newer BaseTool, and to use

the newer features it offers (dependencies, improved file extension

handling, and centralized base command building).
The new version of the tool also handles the case where a .go file

can't be parsed. Previously, we'd log the error but the tool would claim

it passed without issues. The new version parses any error lines it

finds and reports them as comments.

Unit tests pass on Python 2.7 and 3.x.