Cloudfront log importing support and W3CExtendedFormat refactor

A refactor to the W3cExtendedFormat log format that:

1. allows derived classes to add/subtract recognized fields
2. treats the time-taken field as a float field in seconds (as specified by W3C)
3. still treats time-taken as seconds if IIS format is used

Includes support for both normal web Amazon Cloudfront logs and Amazon Cloudfront RTMP logs, also. Multiple spaces are now allowed between fields in the W3C extended log file format.

Note: RTMP logs are logs for Amazon Cloudfront's RTMP streaming service. Reference is here: http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/AccessLogs.html#StreamingDistributionLogFileFormat

RTMP logs contains logs for video playback events. They define some extra fields that for event information. This pull request will recognize those fields and supply them as event tracking query parameters.