./features/playback should be the default directory for playbacks and record_end should save to that directory if PLAYBACK_DIR is not defined

this is potentially a breaking change

i had to do a bunch of recording recently and found it frustrating that `record_end` only saved to the `./` directory, even though i had defined a `PLAYBACK_DIR`.

although i haven't used the xamarin test cloud yet, i am assuming that it behaves like the lesspainful service - you upload your features directory to the cloud along with your app. 

 if you use playbacks, these _must_ be included in the features directory.  the default 'search' directory in `load_playback_data(recording_name, options={})` has been:

`rec_dir = ENV['PLAYBACK_DIR'] || "#{Dir.pwd}/playback"`

which seems strange because most often you are recording from `../features` directory.

putting playbacks in `features/playback` seems like a good convention to follow (and enforce).