added rudimentary support for theming. fixes #67

Conflicts:
	securedrop/example_config.py
add support for theming to journalist interface too (#67)
Merge branch 'theming' of https://github.com/micahflee/securedrop into micahflee-theming
little fixes for custom header image merge

* rename variables to be more descriptive
* use getattr so you don't have to have CUSTOM_HEADER_IMAGE defined in
config.py (so old config files will still work)
* remove some trailing whitespace