Anti-Harvest Download: Serve files for download preventing harvesting

This class can be used to serve files for download preventing harvesting.

It can look a request variable for the name of the file being requested and then performs several checks.

Currently it cleans the file name from dangerous characters by taking the file base name and then it checks if the requested file exists under a given download directory, checks if the file has an accepted file name extension, check if the user browser is not a known robot, and checks if the user address host name is not in a list of known crawler addresses.

The class serves the file for download if all conditions are satisfied.