MOBSDK-482 (Part II)

* AlfrescoContent class created as superclass to AlfrescoContentFile/AlfrescoContentStream
* 2 new constants renamed in InternalConstants: kAlfrescoTypeContent/Folder
* the ObjectConverter translates cmis:document/folder into cm:content/folder
* the DocumentFolderServer translates cm:content/folder back into cmis:document/folder
* the search service allows cm:content/folder in its search statement
* Baseclass test: when we upload the test document we check that the object type is cm:content
* DocumentFolderServiceTest: new test added creating a folder with cm:folder object Type id
* Search service, new test added using cm:content in SQL statement