Add some unit tests

Remove ExecuteAsync extension method from RestSharpExtensions.cs (already available in RestSharp)
Make Msg() extension method not fail if this is null
Add IHttpFactory property to IS24Client
Make BaseXmlSerializer public
Transfered each resource in it's separate class file.
Introduced interfaces for all resources and the clients
Add SerializationTests.cs
Cleanup created code and resolved compiler all warnings
Introduced the IPublishChannelResource which is able to retrieve all user's data on their current available publish channels

This method is not tested, because I'm still missing access rights to the sandbox environment
Introduced delete method for a real estate. This method seems to be possible for real estate objects which are not published
Add more tests
Merge branch 'm4cx-refactoring' of https://github.com/m4cx/IS24RestApi into develop

Conflicts:
	IS24RestApi/IS24Client.cs
Merge remote-tracking branch 'remotes/origin/mganss-test' into develop

Conflicts:
	IS24RestApi/IS24Client.cs
Add more tests
Split tests according to resource types
IAttachmentResource is now implemented as a subresource to IRealEstateResource. This makes the api more natural
Removed commented code
The new method CreateAsync() takes a Stream instance as a parameter. So the API is not restricted on a file system for uploading attachments to IS24.
Added simple tests for ImportExportClient and introduced IResource interface which holds the IIS24Connection
Added new resource for managing the order of uploaded attachments. This resource hasn't been tested yet.
Forgot the ParameterType.UrlSegment on AttachmentsOrder resource
Merge pull request #4 from m4cx/develop

Enhencements and Refactoring
Update XSDs
Fix SampleConsole/Program.cs to work with RealEstateItem
Remove empty MessagesExtensions.cs
Update RestSharp, Rx