Convert README to Markdown.
Add Travis CI build status badge to README.
#2040 - Deactivate modules that no longer exist.
- added module::deactivate_missing_modules()
- revised module::deactivate() to change log message if the module is missing
- added call to new function in module::get_obsolete_modules_message()
- added call to new function when loading admin/maintenance menu
Merge pull request #189 from jozefs/markdown

Convert README to Markdown and show build status badge
Merge pull request #190 from shadlaws/fix_2040

#2040 - Deactivate modules that no longer exist.