Fixed various build and tests issues:
- Added NAnt from NHibernate project (previously ignored by .gitignore)
- Updated NUnit to version 2.6.3
- Signed (strong-named) assemblies
- Decorated ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) with [SecurityCritical] attribute in several implementations
- Changed some build parameters
- Changed SQL Server address for tests to not use SQL Server Express
- Added NuGet.exe
- Updated NHibernate.Validator.nuspec
- Added .editorconfig from NHibernate project
- Updated .gitignore to ignore several files and folders