MOBSDK-556: introducing AlfrescoContentStream

* AlfrescoContentStream class added to Model as per spec
* AlfrescoDocumentFolderService methods using inputStream replaced with methods using contentStream instead.
* AlfrescoDocumentFolderServiceTest: the testCreateVerySmallDocument method is now using contentStream instead of inputStream/mimeType/fileSize
* AlfrescoSiteServiceTest: 2 additional test methods for retrieving pending sites were added (one with, one without listingcontext)