Correct comparison of DateTime in Test
Replace/Rewrite XmlReader/Writer

* XmlReader/Writer is only partially supported on
  portable class libraries and misses some methods
* New approach uses System.Xml.Linq which is in PCL
* Also rename classes to XmlNoteWriter/XmlNoteReader
* Decouple Xml writing from File/Path (not supported in PCL)
* Introduce custom Tomboy.Xml namespace