add logout link if the current session has saved user credentials
fixes XForm viewer, closes #504
fixed caching update mechanism for changed media files, closes #398
Merge pull request #505 from modilabs/image_url_caching_fix

Image url caching fix + form xml viewer fix
rebuilt css (logout link added)
small correction for forms without media, #479
edit in form tester
Merge pull request #174 from modilabs/master

media caching updated, logout links