better placement of specs
clean up have_index matcher:  require exactly one index.  better messages.
bug fix: ActiveRecord 4.* and schema_plus were creating competing indexes for t.references

ActiveRecord 4.* started supporting :index option for t.references, so schema_plus was duplicating that.  fixed by hiding the index options from ActiveRecord and passing them covertly to schema_plus; that way schema_plus’s extended options can still be used (e.g. :index => :unique )

Closes #157