use ruby-units for unit parsing (closes foodcoops/foodsoft#200)

Conflicts:
	Gemfile
	Gemfile.lock

config/initializers/ruby_units.rb

@@ -0,0 +1,37 @@
+# add some more units
+
+if defined? RubyUnits
+  RubyUnits::Unit.redefine!('liter') do |unit|
+    unit.aliases   += %w{ltr}
+  end
+
+  RubyUnits::Unit.redefine!('kilogram') do |unit|
+    unit.aliases   += %w{KG}
+  end
+
+  RubyUnits::Unit.redefine!('gram') do |unit|
+    unit.aliases   += %w{gr}
+  end
+
+  RubyUnits::Unit.define('piece') do |unit|
+    unit.definition = RubyUnits::Unit.new('1 each')
+    unit.aliases    = %w{pc pcs piece pieces}   # locale: en
+    unit.aliases   += %w{st stuk stuks}         # locale: nl
+    unit.kind       = :counting
+  end
+
+  # we use pc for piece, not parsec
+  RubyUnits::Unit.redefine!('parsec') do |unit|
+    unit.aliases = unit.aliases.reject {|u| u=='pc'}
+    unit.display_name = 'parsec'
+  end
+
+  # workaround for ruby-units' require mathn warning: "zero and implicit precision is deprecated."
+  # default precision of 8 which same as all database definitions in db/migrate/20131213002332_*.rb
+  class Rational
+    alias orig_to_d to_d
+    def to_d(precision=8)
+      orig_to_d(precision)
+    end
+  end
+end