Add potrace package

packages/potrace.rb

+require 'package'
+
+class Potrace < Package
+  description 'Potrace(TM) is a tool for tracing a bitmap, which means, transforming a bitmap into a smooth, scalable image.'
+  homepage 'http://potrace.sourceforge.net/'
+  version '1.15'
+  source_url 'http://potrace.sourceforge.net/download/1.15/potrace-1.15.tar.gz'
+  source_sha256 'a9b33904ace328340c850a01458199e0064e03ccaaa731bc869a842b1b8d529d'
+
+  depends_on 'zlibpkg'
+
+  def self.build
+    system './configure'
+    system 'make'
+  end
+
+  def self.install
+    system "make", "DESTDIR=#{CREW_DEST_DIR}", "install"
+  end
+end