Respect DPI settings in text, add pt unit

This has the compatibility impact that all font sizes should be divided by 3 to maintain their same sizes.

Based on PR #225. Thanks @felixleong!!

Conflicts:
	bin/squib
	lib/squib/graphics/text.rb

spec/api/api_units_spec.rb

@@ -14,6 +14,16 @@ describe Squib::Deck do
     end
   end
 
+  context '#pt' do
+    it 'converts inches properly' do
+      expect(deck.points(1)).to eq 4.166666666666667
+    end
+
+    it 'handles strings too' do
+      expect(deck.points('1')).to eq 4.166666666666667
+    end
+  end
+
   context '#cm' do
     it 'converts inches properly' do
       expect(deck.cm(1)).to eq 118.1102361