Add Root path variable & add fix static middleware when app was started not from root folder

Currently, if you use something like monit, god or init.d tool for starting martini application(production server) you will get error because current directory is not root directory of martini application. This pull request will add Root variable and preprocess static middleware folder path. If path is absolute - nothing will happened. if local - root path will be prepended. The same problem right now has `render` middleware, so I think it should be added go core martini library, so than files related additions will reuse this code.

PS was added Godeps for easy dependencies managment