Generate unique indexes for new styles attachments form

Fixes #3940
move gemspec in spree core forward
Spree.user_class class variable accepts a symbol value

Fixes #3930
Timestamps to json give us miliseconds precision

Motivation here was ensure api/orders return updated_at with miliseconds
couldn't find any other easy way to do so. Rails 4 already does it
default
Merge branch 'source/2-0-stable' into 2-0-stable

# By Washington Luiz (2) and others
# Via Washington Luiz
* source/2-0-stable:
  Timestamps to json give us miliseconds precision
  Spree.user_class class variable accepts a symbol value
  move gemspec in spree core forward
  Generate unique indexes for new styles attachments form

Conflicts:
	core/spree_core.gemspec