bugfix for change_instance
refactor behaviour for missing translations

add concept of strategies, and add a sentinel value strategy for callers
to detect missing translations
nicer repr
docs
coverage for repr
docs tweaks
flake
prevent saving of sentinel back to db
allow language to be a callable
flaking
Merge remote-tracking branch 'origin/master' into change_instange_unset_value

Conflicts:
	CHANGES
	setup.py
Merge remote-tracking branch 'origin/change_instange_unset_value' into v0.8.0rc
Merge remote-tracking branch 'origin/callable_language' into v0.8.0rc
Merge remote-tracking branch 'origin/strategy_for_missing' into v0.8.0rc

Conflicts:
	taal/__init__.py
changes + version
Merge pull request #40 from onefinestay/v0.8.0rc

0.8.0 rc