add Encounter model.

app/models/encounter.rb

@@ -0,0 +1,7 @@
+class Encounter
+  include Mongoid::Document
+
+  belongs_to :field
+
+  field :probability, type: Integer, default: 0
+end

spec/factories/encounters.rb

@@ -0,0 +1,6 @@
+# Read about factories at https://github.com/thoughtbot/factory_girl
+
+FactoryGirl.define do
+  factory :encounter do
+  end
+end

spec/models/encounter_spec.rb

@@ -0,0 +1,5 @@
+require 'spec_helper'
+
+describe Encounter do
+  pending "add some examples to (or delete) #{__FILE__}"
+end