Use w3lib to generate feed uris.

The file path feed uri generation was producing invalid relative feed:
uris.  w3lib has a method for generating qualified feed uris, so I used
that here.

Example:
The feed path "items/1/spider/hash.jl" would be converted to feed uri
"file://items/1/spider/hash.jl", which actually represents path
"/1/spider/hash.jl".
Merge pull request #73 from andrewbaxter/feeduri_w3lib

Use w3lib to generate feed uris.