Add handling of dicom references

Created by: fedorov

This topic has been discussed at SLC2015 Project Week, see: http://www.na-mic.org/Wiki/index.php/2015_Winter_Project_Week:DICOM_References

The implementation follows the plan outlined during the project week.

The idea is the following:

1. plugins will populate referencedInstanceUIDs in loadable during Examine
2. on Load, referencedInstanceUIDs corresponding to the selected loadables is checked, and the files matching those UIDs are located
3. plugins selected (by the user) are invoked to identify loadables corresponding to the referenced file lists
4. popup dialog with the list of identified loadables (only those 'selected' by plugins are considered) and the checkboxes allowing the user to cancel loading
5. when the user hits 'Done' button in the popup, selected referenced loadables are added to the pluginsByLoadable dictionary, and all loadables are loaded per usual procedure

The proposed patch has been reviewed by @pieper and @cpinter.