[432] Add middleware for exception logging

Add middleware class that logs exceptions. This can be used on the live
server to get stack traces for 500 errors.
[#432] Settings changes to logging

In 10-base.conf:
  Add ExceptionLoggingMiddleware to MIDDLEWARE_CLASSES

  Create settings for the paths to and names of the logging files,
  akvo.log and akvo.error.log. LOG_FILE_PATH is now set to the "default"
  path used in the vagrant setup.

  Remove LOGGING

In 90-finish.conf:
  Add ACCESS_LOG_FILE_FULL_NAME and ERROR_LOG_FILE_FULL_NAME

  Edit LOGGING to support the new exception handling middleware logger

  Also some cleanup of the LOGGING setting. It will need more work when
  we start using logging more in earnest.
[#432] Fix default path to log files
Merge pull request #536 from akvo/feature/432_live_server_logging

[#432] live server logging