Add ctkDICOMUtil and ::setDICOMLogLevel helper

This function allows to set the DICOM log level for
diagnostic purposes.
Define ctkErrorLogLevel::logLevelAsString as static for easier re-use

Explicitly add associated decorator. See code comment for explanation.
Re-factor ctkDICOMUtil to be consistent with ctkUtil

Add the methods dicomLogLevel and setDICOMLogLevel to "ctk" namespace

Update PythonQt external project to include fix allowing to wrap
QFlags.

The DICOM log level can be set from python doing:

 >>> import ctk
 >>> ctk.ctk.dicomLogLevel()
 16
 >>> ctk.ctk.dicomLogLevelAsString()
 u'Info'
 >>> ctk.ctk.setDICOMLogLevel(ctk.ctkErrorLogLevel.Critical)
 >>> ctk.ctk.dicomLogLevel()
 256
 >>> ctk.ctkErrorLogLevel.logLevelAsString(ctk.ctk.dicomLogLevel())
 u'Critical'
 >>> ctk.ctkErrorLogLevel.logLevelAsString(32)
 u'Warning'
 >>> ctk.ctk.setDICOMLogLevel(64)
 >>> ctk.ctk.dicomLogLevelAsString()
 u'Error'
Merge branch 'add_dicomutil'

* add_dicomutil:
  Re-factor ctkDICOMUtil to be consistent with ctkUtil
  Define ctkErrorLogLevel::logLevelAsString as static for easier re-use
  Add ctkDICOMUtil and ::setDICOMLogLevel helper