Use abort instead of stderr + exit.

Abort writes to stderr and exits afterwards; thus, abort is a suitable
substitute for the two combinations that were previously there.
github.rb: use abort() in more places

And remove an extra exit()
Call abort in die. Replace abort with die.
Switch to empty string instead of nil.
git.rb: add punctuation to separate clauses
Merge use-abort-instead-of-die (#76) into master

Use abort instead of stderr + exit.

Abort writes to stderr and exits afterwards; thus, abort is a suitable
substitute for the two combinations that were previously there.

Conflicts:
	lib/github.rb