Adding :none sort mode to enumeration

Hi there.
After you closed this (https://github.com/cassiomarques/enumerate_it/pull/30) i realized that the thing i needed - wasn't really introduced. In my app i actually wanted to return enumeration in order I used when invoking associate_values. So i kind of added another sort_mode :none to the list of usable modes. I guess it might not work in ruby 1.8, but as of 1.9 hashes preserve key order - it will work for me.
One thing to consider - maybe don't do sorting at all if :none is set, but for now i just integrated this option into the working code.
Also i changed tests a bit, so that none of sort results repeat.