Replace use of ::I18n.locale with context.locale

This change makes the locale setting thread-safe.

The right way to use it is to pass "force_locale: context.locale"
or to pass context.locale as third argument to I18n's translate()
and locale() methods.

Also, remove last remaining pieces of dependence on Amber, since the
shard is generic.

spec/citrine-i18n_spec.cr

@@ -13,7 +13,7 @@ describe Citrine::I18n::Handler do
     context = create_context(request)
     handler = Citrine::I18n::Handler.new
     handler.call(context)
-    I18n.locale.should eq "fr"
+    context.locale.should eq "fr"
   end
 
   it "should set language from complicated header" do
@@ -22,7 +22,7 @@ describe Citrine::I18n::Handler do
     context = create_context(request)
     handler = Citrine::I18n::Handler.new
     handler.call(context)
-    I18n.locale.should eq "en"
+    context.locale.should eq "en"
   end
 
 end

src/citrine-i18n.cr

@@ -1,10 +1,14 @@
-require "amber"
+require "http/server/handler"
 require "i18n"
 require "./citrine-i18n/parser"
 require "./citrine-i18n/handler"
 
+class HTTP::Server::Context
+  property locale : String = "en"
+end
+
 module Citrine::I18n
-	def self.configure
-		yield ::I18n.config
-	end
+  def self.configure
+    yield ::I18n.config
+  end
 end

src/citrine-i18n/handler.cr

@@ -9,7 +9,7 @@ module Citrine
         if languages = context.request.headers[HEADER]?
           parser = Citrine::I18n::Parser.new languages
           compat = parser.compatible_language_from ::I18n.available_locales
-          ::I18n.locale = compat if compat
+          context.locale = compat if compat
           #Amber.logger.debug "Languages available: #{languages.to_s}"
           #Amber.logger.debug "Language chosen: #{::I18n.locale}"
         end