Added support for downloading by filename

Changed how filenames are handled, still needs some tweaking, and allows users to redownload based on filename.
Deleting prebuilt binary, fixed filename formatting.

There was a problem with how the filenames were being formatted in the method I created. This has been fixed to match the original filename of the pixiv url.
Added login to grab r18 images within Illustration

Under certain circumstances, Illustration will need to download stuff while logged in for r18 content. I created a public HttpWebResponse in Illustration.cs and a public cookie in Pixiv.cs to hack-fix this problem.
Merge pull request #2 from cglmrfreeman/master

Merge