Improved error and exception handling.
Removed warnings.
Renamed XNAT classes.

- ctkXnatServer to ctkXnatDataModel
- ctkXnatConnection to ctkXnatSession
- ctkXnatConnectionFactory to ctkXnatSessionFactory
Moved the ctkXnatLoginProfile class to XNAT/Core.
ctkXnatSession now requires a ctkXnatLoginProfile in its constructor.
Moved fetch() methods to subclasses and use QMetaType to create instances.
Added http session support.
Removed debugging messages.
Re-structured XNAT exception hierarchy.
Added ctkXnatSession documentatioin.
Make schema type customizable per object again.
Updated qRestAPI commit hash