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.
HACKING: tests on python3 require rpm-python3
resolve another use of commands module for python3 compat

03255001d6 moved commands module to src/tito/compat.py to handle
differences between python2 and python3, but missed a bit that
did not show up in unit or functional tests.

Fix those occurences and add unit test to detect the regression.
deprecate GitPython

GitPython is not compatible with python3 and may never be:
https://fedoraproject.org/wiki/User:Churchyard/python3
Merge pull request #109 from jumanjiman/deprecate_gitpython

Deprecate GitPython
Fix MockBuilder bug from refactoring.
Functional test for the CoprReleaser.
Print output live for longer running rpmbuild commands.
Use test util to write files.
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.