Enabling ESRI for storing results

- `api_key` param
- `forStorage` param
- token generation
- updating README.md

README.md

@@ -568,15 +568,15 @@ The [Google Places Details API](https://developers.google.com/places/documentati
 
 #### ESRI (`:esri`)
 
-* **API key**: none
+* **API key**: optional (set `Geocoder.configure(:esri => {:api_key => ["client_id", "client_secret"]})`)
 * **Quota**: Required for some scenarios (see Terms of Service)
 * **Region**: world
 * **SSL support**: yes
 * **Languages**: English
-* **Documentation**: http://resources.arcgis.com/en/help/arcgis-online-geocoding-rest-api/
+* **Documentation**: https://developers.arcgis.com/rest/geocode/api-reference/overview-world-geocoding-service.htm
 * **Terms of Service**: http://www.esri.com/legal/software-license
-* **Limitations**: ?
-* **Notes**: You can specify which projection you want to use by setting, for example: `Geocoder.configure(:esri => {:outSR => 102100})`.
+* **Limitations**: Requires API key if results will be stored. Using API key will also remove rate limit.
+* **Notes**: You can specify which projection you want to use by setting, for example: `Geocoder.configure(:esri => {:outSR => 102100})`. If you will store results, set the flag and provide API key: `Geocoder.configure(:esri => {:api_key => ["client_id", "client_secret"], :for_storage => true})`
 
 #### Mapzen (`:mapzen`)
 

lib/geocoder/lookups/esri.rb

@@ -41,8 +41,29 @@ module Geocoder::Lookup
       else
         params[:text] = query.sanitized_text
       end
+      params[:token] = token if configuration.api_key
+      params[:forStorage] = configuration.for_storage if configuration.for_storage
       params.merge(super)
     end
 
+    def token
+      unless token_is_valid
+        getToken = Net::HTTP.post_form URI('https://www.arcgis.com/sharing/rest/oauth2/token'),
+          f: 'json',
+          client_id: configuration.api_key[0],
+          client_secret: configuration.api_key[1],
+          grant_type: 'client_credentials',
+          expiration: 1440 # valid for one day,
+
+          @token = JSON.parse(getToken.body)['access_token']
+          @token_expires = Time.now + 1.day
+      end
+      return @token
+    end
+
+    def token_is_valid
+      @token && @token_expires > Time.now
+    end
+
   end
 end