Patch for email notification of download

Firstly, thank you for this module - it has been very useful to handle tracking of downloads on my company's site. One thing I was asked was whether it would be possible to generate an email notification whenever a file was downloaded. I did this, and have had to extend the configuration options to minimise the email traffic. Attached is a patch to the 5.x-1.x-dev version.

Added configuration options:

Checkbox: email site owner on download;
textbox: list of sites to ignore.

The list of sites to ignore is to avoid sending emails when search-engine robots access the site, at the moment it is a simple list of space-separated strings, if any of those are present in the downloader's address, then the download is not counted.

Please let me know if this is useful, or of any way this can be improved. My PHP coding skills are very small, so this could probably be done better!