Add support for an :exclude option in Buildfiles.

The addition of the PhantomJS test runner exposed the fact that there
isn't a good way to exclude parts of a target from being included in a
build. So I added a new :exclude option that can be used to tell Abbot
to exclude directories from a build.
Add default values for whitelist/blacklist/accept files.

While fixing the previous issue, I noticed that the help string for
whitelist/blacklist/accept mentions a default value, but there was no
default value set for these options.
Merge pull request #101 from FernCreek/patch/fairbanksg/buildfile-exclude

Adds exclude option to abbot.