make api_url as configurable

hereby it is able to:
1. refer to different api urls (beta | api)
2. opt for protocol less urls to work with ssl connection

lib/url2png/config.rb

@@ -3,6 +3,7 @@ module Url2png
 
   # modes
   MODES = %w{production placehold dummy}
+  API_URL = 'http://beta.url2png.com'
 
   def config c = {}
     # mandatory
@@ -12,6 +13,7 @@ def config c = {}
     # optional
     self.mode = c[:mode] if c[:mode]
     self.api_version = c[:api_version] if c[:api_version]
+    self.api_url = c[:api_url] if c[:api_url]
   end
 
   def api_key=api_key
@@ -49,6 +51,16 @@ def api_version
     @api_version || 'v6' #default: v6
   end
 
+  def api_url=api_url
+    @api_url = api_url || API_URL
+  end
+
+  def api_url
+    # reference => http://url2png.com/docs/
+    # currently all versions suggest 'beta'
+    @api_url || API_URL
+  end
+
   def default_size=default_size
     @default_size = default_size || "400x400"
   end

lib/url2png/helpers/common.rb

@@ -111,8 +111,7 @@ def url2png_image_url url, options = {}
 
             # set the format of the asset requested
             format = options[:format] || "png"
-
-            "http://api.url2png.com/v6/#{Url2png.api_key}/#{token}/#{format}/?#{query_string}"
+            "#{ Url2png.api_url }/v6/#{Url2png.api_key}/#{token}/#{format}/?#{query_string}"
 
           when 'v4'
             ######
@@ -144,7 +143,7 @@ def url2png_image_url url, options = {}
 
             # build image url
             File.join(
-              "http://beta.url2png.com",
+              "#{Url2png.api_url}",
               Url2png.api_version,
               Url2png.api_key,
               token,
@@ -180,7 +179,7 @@ def url2png_image_url url, options = {}
 
             # build image url
             File.join(
-              "http://api.url2png.com",
+              "#{Url2png.api_url}",
               Url2png.api_version,
               Url2png.api_key,
               token,