Refactored Downloadable Files Import

[sprankhub]

• imported downloaded files are now created via the standard uploader Mage_ImportExport_Model_Import_Uploader
• this ensures that these files are created in sub-folders like /media/downloadable/files/links/f/i/file.pdf
• this also ensures that these folders will be created if they do not exist yet

[sprankhub]

Thanks for your review! Please see my last commits. I agree that the usage of @ should really be avoided. To be honest I basically simply copied the method _getUploader, which does exactly the same. Hence, it is a more general problem. I created a new issue #394 in order to work on this.

Test data can be taken form here. I will try to write test scripts and/or update the wiki and/or the website as soon as my work on downloadable products is finished.

[sprankhub]

@nhp Would you approve the changes after my updates? Thanks!

[sprankhub]

@nhp would you have a short look again? Thanks!