Add support for ascii output with various symbol amount

The support for different chars in the output array was former
restricted to 10. But this is not necessary in the code. So I changed
that.
As example I've added the alphabet.

lib/rubella/output/ascii.rb

@@ -37,6 +37,10 @@ module Rubella
                   [" ", "·", ",", ";", "o", "O", "%", "8", "@", "#"]
         @symbols["numbers"]      =
                   [" ", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
+        @symbols["letters"]      =
+                  [" ", "a", "b", "c", "d", "e", "f", "g", "h",
+                   "i", "j", "k", "l", "m", "n", "o", "p", "q",
+                   "r", "s", "t", "u", "v", "w", "x", "y", "z"]
 
         self.used_symbols = "shades_ascii"
         super data, field_size
@@ -81,12 +85,12 @@ module Rubella
         @data.each do |point|
           i = 0
           point.reverse.each do |part|
-            part = (part*10).to_i       
+            part = (part*@symbols[@used_symbols].length).to_i
 
             # Fix to prevent possible overflow.. should never happen, but we
             # are careful
-            if part > 9
-              part = 9
+            if part > (@symbols[@used_symbols].length-1)
+              part = (@symbols[@used_symbols].length-1)
             end
 
             ascii_arr[i] << @symbols[@used_symbols][part]