Merge pull request #28 from gumatias/master

adding added_at and added_by fields to playlist
reducing duplicate code
breaking long lines
Time::parse will break with nil arg
added_by should return user instead of user id
modularizing and giving meaningful var names
documenting new playlist instance vars
specs for playlist's tracks_added_at and tracks_added_by
moving tests section in README
making User#to_hash more concise
bump to 1.9.0