Add GitAnnexBuilder, using git-annex to store blobs

This is an extension of NoTgzBuilder, which instead of storing blobs in source
control directly, allows the use of git-annex to reference blobs in an external
source (e.g. a web URL remote).  When preparing sources, it uses git-annex's
unlock command to temporarily restore the original file.
Merge branch 'git-annex' of https://github.com/domcleal/tito into domcleal-git-annex

Conflicts:
	src/tito/builder/__init__.py
	src/tito/builder/main.py
Fix some git annex merge issues.
Add test for GitAnnexBuilder.