Data annotation changes and mvc4 demo project updates

# PostBuild
Added functionality to transform view model annotations using regex, from ```[Display(Name = "Test1", Prompt="Test2", ShortName="Test3")]``` 
to ```[Display(_("Test1"), _("Test2"), _("Test3"))]``` so that the annotations can be processed by xgettext, after which point the annotations are restored to their original values.

Updated post build to throw more granular exceptions if gettext folder is missing
Updated readme to include details regarding setup of the gettext folder

# I18n:
Added DataAnnotationsModelMetadataProvider to fix issue with display attribute not being displayed in mvc view
Fixed required and string length attributes so that they they generate client side validation in mvc views.
Fixed issue in post build where manifest is not deleted if there is an exception in post build execute.
Fix ```LocalizingService.GetTextOrDefault``` so that it doesn't crash if localizable annotation property is null.

# MVC4 Demo
Add view model with translatable annotations
Display model in view
Add action to set language in session
Add test strings in pl (polish) translation file
Add web config to locale folder so its not externally accessible
Add translatable string in views, models and controllers
set base view page for razor views to be I18NWebViewPage
Added generated po files
use I18nModelMetadataProvider
Disable automatic routing (pending a fix)