Take a look here:
You should add
helper.label_class = 'col-lg-x' #for example 'col-lg-2'
helper.field_class = 'col-lg-x' #for example 'col-lg-10'
BS3 has a 12 column grid system, more info about that here http://getbootstrap.com/css/#grid
That made the form horizontal but unfortunately the labels align to the left, I'm trying to fix that now.
UPDATE: If you have issues with the vertical spacing between the fields add a row css class to the outer div of every field, the html output:
<div class="form-group row" id="div_id_title">
###labels HTML###
</div>
Probably a bug on the field.html template for bootstrap3.