Use Log4r instead of the default Rails logger.

This gives us far greater flexibility to configure logging like this:

- proper timestamp (ISO8601) in every log line
- log level printed to every log line
- staging and production environments log to daily rolling files
- test and development environments log to a file which is truncated on every launch
- development environment additionally logs to stdout (for easier visualization of logs when launching "rails server")
Included Bootstrap responsive styles.
Style flash messages with Bootstrap in application layout. Moved bootstrap javascript include to AFTER including jquery, otherwise bootstrap javascript doesn't work (for instance closing alerts).
Started internationalizing Devise views.
Moved i18n strings for the Devise views to their own yml files.
Merge branch 'pretty-devise' of github.com:amatriain/openreader into pretty-devise