Download plugin: Temp directory and test cases

Alright, second attempt, slowly getting the hang of git.. :|

I've been using this myself because Flexget is on my SSD and I wanted to avoid unnecessary write cycles.
Turns out, it's also a [3-year old ticket](http://flexget.com/ticket/452) on Trac :)
Although my commit only allows for a change of the temp directory, not disable it altogether. IMO that would require a bigger rewrite of the download plugin with the only benefit being the ability maybe to watch a movie while it downloads - which would break again once multithreaded downloads are allowed.