Fixed up the sql query for returning all file downloads for a user.
Commented out all mentions of file expiry, as we're not supporting this functionality right now.

Changed permission from 'hide upload info checkboxes' to 'show upload info checkboxes'
Added a user/myfiles menu item which redirects the user to their purchased files, at user/%user/store/myfiles
Some small whitespace fixes.
Cleaned up ec_file_file_download() which works as follows:
* If file is sellable and the user owns the node which is being viewed, grant access.
* If user has bought the file, grant access.

Added more descriptive info about how the checkboxes work to the README.