ENH: Slice view annotations

@pieper, @fedorov
This is an implementation of slice view annotations in DataProbe as a part of QIICR Project [1]. It includes the following features:
* Corner text annotations including DICOM annotations.
* Color scalar bar for background volume
* Scaling ruler

This is a working script, however we would also try to add the following extra features on Wednesday:
* Smarter logic for
 *  Height of scalar bar
 *  Maximum length of corner annotation lines
 *  Hiding of the top right corner annotation

Some Known Issues:
* We could not add the color scalar bar for foreground as there is no method in vtkMRMLSliceLogic
    to get Foreground window level and range ( for background we use GetBackgroundWindowLevelAndRange)

* There is a random behavior which makes color scalar bars to disappear or placed at random positions. It has something to do with multi-sampleing option as deleting the MSAA flag in Slicer settings file (*.ini) fixes this.

* There is also a bug in Windows that makes the annotations texts not readable on the white background (it looks like that the shadowing does not work correctly). It is introduced recently so I am suspicious to either MSAA or vtk6. 

* In color scalar bar there is no control on the font size of the label. One solution can be to use vtkPVScalarBarActor [2] from paraview.


[1] [QIICR Slicer infrastructure projects](https://github.com/QIICR/ProjectIssuesAndWiki/wiki/Slicer-Infrastructure-Projects)
[2] http://www.paraview.org/ParaView3/Doc/Nightly/html/classvtkPVScalarBarActor.html