A few more artifacts addon tweaks based on @joshk conversation :smiley_cat:
Breaking out non-runnable messaging, constantizing some settings for artifacts addon
Raising artifacts max-size to 50MB, handling branch config better

per feedback on #242
Merge branch 'artifacts-support' of git://github.com/meatballhat/travis-build into meatballhat-artifacts-support

Conflicts:
	lib/travis/build/script/addons/artifacts.rb