Load English messages from default theme.

Theme might not provide default English messages, and if it does, theme path
is the first to be searched during the import.
Date format setting was not used.
Merge pull request #300 from kotnik/dev

Misc fixes
updated changelog
first very basic integration test
first very basic integration test
don't leave the test site there
Fix #302 (with tests)
cheaper tests
correct naming
Fix Issue #298
Merge remote-tracking branch 'upstream/master' into my_branch

Conflicts:
	nikola/plugins/task_render_galleries.py