Fix symlinks (relative link, argument order, folder creation)

[raphCode]

Was completely broken basically:

• No parent directories were created
• The link target was wrong and not a local path
• The symlink function was called with source and target swapped (this is a documentation
  bug in the symlink crate tho)

[codecov]

Codecov Report

Merging #182 (fd6764e) into master (84276b9) will decrease coverage by 0.92%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   64.73%   63.80%   -0.93%     
==========================================
  Files          17       17              
  Lines         621      630       +9     
==========================================
  Hits          402      402              
- Misses        219      228       +9     

Impacted Files	Coverage Δ
src/disk.rs	0.00% <0.00%> (ø)

[Skallwar]

Can you give me the command you used to test this?

[raphCode]

suckit http://fusor.net --jobs 16 --verbose --exclude-download '(?:view=print)|(?:viewtopic\.php.*p=)|(?:posting|search|ucp|viewonline|memberlist)\.php|hilit=' --tries 50 --visit-filter-is-download-filter

Basically any phpBB forum will do.
The downloaded attachment files end up in the local root output directory, while the original links are transformed into symlinks, for example:
fusor.net/board/download/file.php__querystring__avatar=1031_1367752737.jpg -> ../../../UTF-8''1031.jpg