Remove singleton DiskStorage.Instance

This shouldn't be static in the library. If a client can
safely assume only to have a single, main storage,
it should be a singleton in the client, but not in the
library.
Rename SyncObjects to SyncManifest
Remove XmlTextReader

* Use XmlReader / XmlWriter instead which are supported for portable
  class libraries
Add simple logging
Introduce simple IoC container

* Using PortableIoC that is very tiny, PCL compatible,
  and single file
Replace XmlWriter/XmlReader for SyncManifest

* Use System.Xml.Linq instead (supported by PCL)
* XmlReader/Writer has issues on PCL, i.e. ReadString()
  is not supported
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
Remove references to .ForEach

* .ForEach is removed in PCL and official suggested
  action is to replace with regular foreach construct.
Fix custom namespace for XmlNoteWriter
Drop DevDefined.OAuth dependency

* Do all OAuth client stuff ourselves
* Pulled classes from existing tomboy for linux code
Remove DevDefined.OAuth git submodule
Add helper method to perform OAuth token exchange
Fix in-place changing of serverUrl
HttpUtility.UrlEncode => Uri.EscapeUriString

* HttpUtility not included in PCL
Replace Header.Add with indexer access

For some reason, PCL only supports indexer access and
has no .Add () method
Remove crypto classes for OAuth

* Use PLAINTEXT OAuth only since PCL has limited crypto
* This is non fatal, as we (a) rely on SSL and (b) Rainy doesn't
  check for signatures and (c) signatures don't add security if
  the ConsumerKey is publicly known as in our case ("anyone")
Replace HttpWebRequest with HttpClient

* PCL has no System.Web
Use IoC for Logger registration
Ignore OAuth tests by default

Although tests work, it might not be a good
idea to have it enabled by default to keep
demo server load to a minimum