First set of build system patches for Qt5 support.

These changes enable support for Qt5 in CTK libraries and CTK Qt plugins.
Set the CTK_USE_QT5 CMake variable to ON on specify the Qt5 installaltion
directory in QT5_INSTALL_PREFIX.
This commit enables Qt5 support for the CTK Core library, issue #277.

There is still a Qt5Widgets dependency, see issue #278. The tests
pass (on my Ubuntu 12.04 development machine) with both Qt4 and Qt5.
Explicitly include the Qt use file again.
Added qRestAPI as an external project.
Moved the XNAT clases from the org.commontk.xnat plug-in.
Moved the XNAT classes into a Core subdirectory.
Added XNATWidgets library stub.
Removed unused XNAT files and simplified QtScript dependency.
Added initial ctkXNATBrowser application.
Login dialog added
qRestAPI upgrade
Fix for compiler warnings
ctkXnatObject redesign. No QObjects, using QSharedPointer.
Qt list model for ctkXnatObject instances. Will be renamed.
Prototype application for querying XNAT servers.
Add support for Qt5

Todo:
* Test with Qt4
* Make sure ctkMacroWrapPythonQt.cmake works correctly
Ensure edit line in ctkConsole is always visible

Even when the line is 2 line long.
Also, on Mac, typing Control (not Command) was resetting the cursor
to be at the end of the document.

Closes #295
Add open file dialog and print help message actions

Closes #315
ctkXnatConnection* moved from ctkXnatObject to ctkXnatServer
XNAT tree model and example application